diff --git a/src/BaseSeed.php b/src/BaseSeed.php
index 85532272..cc0e1728 100644
--- a/src/BaseSeed.php
+++ b/src/BaseSeed.php
@@ -191,6 +191,16 @@ public function insertOrSkip(string $tableName, array $data): void
         $table->insertOrSkip($data)->save();
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public function insertOrUpdate(string $tableName, array $data, array $updateColumns, array $conflictColumns): void
+    {
+        // convert to table object
+        $table = new Table($tableName, [], $this->getAdapter());
+        $table->insertOrUpdate($data, $updateColumns, $conflictColumns)->save();
+    }
+
     /**
      * {@inheritDoc}
      */
diff --git a/src/Db/Adapter/AbstractAdapter.php b/src/Db/Adapter/AbstractAdapter.php
index 7251ba31..f8d14125 100644
--- a/src/Db/Adapter/AbstractAdapter.php
+++ b/src/Db/Adapter/AbstractAdapter.php
@@ -622,9 +622,14 @@ public function fetchAll(string $sql): array
     /**
      * @inheritDoc
      */
-    public function insert(TableMetadata $table, array $row, ?InsertMode $mode = null): void
-    {
-        $sql = $this->generateInsertSql($table, $row, $mode);
+    public function insert(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
+        $sql = $this->generateInsertSql($table, $row, $mode, $updateColumns, $conflictColumns);
 
         if ($this->isDryRunEnabled()) {
             $this->io->out($sql);
@@ -649,10 +654,17 @@ public function insert(TableMetadata $table, array $row, ?InsertMode $mode = nul
      * @param \Migrations\Db\Table\TableMetadata $table The table to insert into
      * @param array $row The row to insert
      * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on upsert conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness for upsert (unused in MySQL)
      * @return string
      */
-    protected function generateInsertSql(TableMetadata $table, array $row, ?InsertMode $mode = null): string
-    {
+    protected function generateInsertSql(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): string {
         $sql = sprintf(
             '%s INTO %s ',
             $this->getInsertPrefix($mode),
@@ -667,8 +679,10 @@ protected function generateInsertSql(TableMetadata $table, array $row, ?InsertMo
             }
         }
 
+        $upsertClause = $this->getUpsertClause($mode, $updateColumns, $conflictColumns);
+
         if ($this->isDryRunEnabled()) {
-            $sql .= ' VALUES (' . implode(', ', array_map($this->quoteValue(...), $row)) . ');';
+            $sql .= ' VALUES (' . implode(', ', array_map($this->quoteValue(...), $row)) . ')' . $upsertClause . ';';
 
             return $sql;
         } else {
@@ -680,7 +694,7 @@ protected function generateInsertSql(TableMetadata $table, array $row, ?InsertMo
                 }
                 $values[] = $placeholder;
             }
-            $sql .= ' VALUES (' . implode(',', $values) . ')';
+            $sql .= ' VALUES (' . implode(',', $values) . ')' . $upsertClause;
 
             return $sql;
         }
@@ -701,6 +715,29 @@ protected function getInsertPrefix(?InsertMode $mode = null): string
         return 'INSERT';
     }
 
+    /**
+     * Get the upsert clause for MySQL (ON DUPLICATE KEY UPDATE).
+     *
+     * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness (unused in MySQL)
+     * @return string
+     */
+    protected function getUpsertClause(?InsertMode $mode, ?array $updateColumns, ?array $conflictColumns = null): string
+    {
+        if ($mode !== InsertMode::UPSERT || $updateColumns === null) {
+            return '';
+        }
+
+        $updates = [];
+        foreach ($updateColumns as $column) {
+            $quotedColumn = $this->quoteColumnName($column);
+            $updates[] = $quotedColumn . ' = VALUES(' . $quotedColumn . ')';
+        }
+
+        return ' ON DUPLICATE KEY UPDATE ' . implode(', ', $updates);
+    }
+
     /**
      * Quotes a database value.
      *
@@ -748,9 +785,14 @@ protected function quoteString(string $value): string
     /**
      * @inheritDoc
      */
-    public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode = null): void
-    {
-        $sql = $this->generateBulkInsertSql($table, $rows, $mode);
+    public function bulkinsert(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
+        $sql = $this->generateBulkInsertSql($table, $rows, $mode, $updateColumns, $conflictColumns);
 
         if ($this->isDryRunEnabled()) {
             $this->io->out($sql);
@@ -785,10 +827,17 @@ public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode
      * @param \Migrations\Db\Table\TableMetadata $table The table to insert into
      * @param array $rows The rows to insert
      * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on upsert conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness for upsert (unused in MySQL)
      * @return string
      */
-    protected function generateBulkInsertSql(TableMetadata $table, array $rows, ?InsertMode $mode = null): string
-    {
+    protected function generateBulkInsertSql(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): string {
         $sql = sprintf(
             '%s INTO %s ',
             $this->getInsertPrefix($mode),
@@ -799,11 +848,13 @@ protected function generateBulkInsertSql(TableMetadata $table, array $rows, ?Ins
 
         $sql .= '(' . implode(', ', array_map($this->quoteColumnName(...), $keys)) . ') VALUES ';
 
+        $upsertClause = $this->getUpsertClause($mode, $updateColumns, $conflictColumns);
+
         if ($this->isDryRunEnabled()) {
             $values = array_map(function ($row) {
                 return '(' . implode(', ', array_map($this->quoteValue(...), $row)) . ')';
             }, $rows);
-            $sql .= implode(', ', $values) . ';';
+            $sql .= implode(', ', $values) . $upsertClause . ';';
 
             return $sql;
         } else {
@@ -820,7 +871,7 @@ protected function generateBulkInsertSql(TableMetadata $table, array $rows, ?Ins
                 $query = '(' . implode(', ', $values) . ')';
                 $queries[] = $query;
             }
-            $sql .= implode(',', $queries);
+            $sql .= implode(',', $queries) . $upsertClause;
 
             return $sql;
         }
diff --git a/src/Db/Adapter/AdapterInterface.php b/src/Db/Adapter/AdapterInterface.php
index 15c30fe9..0b60f059 100644
--- a/src/Db/Adapter/AdapterInterface.php
+++ b/src/Db/Adapter/AdapterInterface.php
@@ -480,9 +480,17 @@ public function fetchAll(string $sql): array;
      * @param \Migrations\Db\Table\TableMetadata $table Table where to insert data
      * @param array $row Row
      * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on upsert conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness for upsert
      * @return void
      */
-    public function insert(TableMetadata $table, array $row, ?InsertMode $mode = null): void;
+    public function insert(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void;
 
     /**
      * Inserts data into a table in a bulk.
@@ -490,9 +498,17 @@ public function insert(TableMetadata $table, array $row, ?InsertMode $mode = nul
      * @param \Migrations\Db\Table\TableMetadata $table Table where to insert data
      * @param array $rows Rows
      * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on upsert conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness for upsert
      * @return void
      */
-    public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode = null): void;
+    public function bulkinsert(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void;
 
     /**
      * Quotes a table name for use in a query.
diff --git a/src/Db/Adapter/AdapterWrapper.php b/src/Db/Adapter/AdapterWrapper.php
index a291db0f..f8dd0480 100644
--- a/src/Db/Adapter/AdapterWrapper.php
+++ b/src/Db/Adapter/AdapterWrapper.php
@@ -138,17 +138,27 @@ public function query(string $sql, array $params = []): mixed
     /**
      * @inheritDoc
      */
-    public function insert(TableMetadata $table, array $row, ?InsertMode $mode = null): void
-    {
-        $this->getAdapter()->insert($table, $row, $mode);
+    public function insert(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
+        $this->getAdapter()->insert($table, $row, $mode, $updateColumns, $conflictColumns);
     }
 
     /**
      * @inheritDoc
      */
-    public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode = null): void
-    {
-        $this->getAdapter()->bulkinsert($table, $rows, $mode);
+    public function bulkinsert(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
+        $this->getAdapter()->bulkinsert($table, $rows, $mode, $updateColumns, $conflictColumns);
     }
 
     /**
diff --git a/src/Db/Adapter/PostgresAdapter.php b/src/Db/Adapter/PostgresAdapter.php
index 6ec16ce0..165cfa21 100644
--- a/src/Db/Adapter/PostgresAdapter.php
+++ b/src/Db/Adapter/PostgresAdapter.php
@@ -1152,8 +1152,13 @@ public function setSearchPath(): void
     /**
      * @inheritDoc
      */
-    public function insert(TableMetadata $table, array $row, ?InsertMode $mode = null): void
-    {
+    public function insert(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
         $sql = sprintf(
             'INSERT INTO %s ',
             $this->quoteTableName($table->getName()),
@@ -1172,7 +1177,7 @@ public function insert(TableMetadata $table, array $row, ?InsertMode $mode = nul
             $override = self::OVERRIDE_SYSTEM_VALUE . ' ';
         }
 
-        $conflictClause = $this->getConflictClause($mode);
+        $conflictClause = $this->getConflictClause($mode, $updateColumns, $conflictColumns);
 
         if ($this->isDryRunEnabled()) {
             $sql .= ' ' . $override . 'VALUES (' . implode(', ', array_map($this->quoteValue(...), $row)) . ')' . $conflictClause . ';';
@@ -1198,8 +1203,13 @@ public function insert(TableMetadata $table, array $row, ?InsertMode $mode = nul
     /**
      * @inheritDoc
      */
-    public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode = null): void
-    {
+    public function bulkinsert(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
         $sql = sprintf(
             'INSERT INTO %s ',
             $this->quoteTableName($table->getName()),
@@ -1215,7 +1225,7 @@ public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode
 
         $sql .= '(' . implode(', ', array_map($this->quoteColumnName(...), $keys)) . ') ' . $override . 'VALUES ';
 
-        $conflictClause = $this->getConflictClause($mode);
+        $conflictClause = $this->getConflictClause($mode, $updateColumns, $conflictColumns);
 
         if ($this->isDryRunEnabled()) {
             $values = array_map(function ($row) {
@@ -1258,14 +1268,30 @@ public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode
      * Get the ON CONFLICT clause based on insert mode.
      *
      * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on upsert conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness for upsert
      * @return string
      */
-    protected function getConflictClause(?InsertMode $mode = null): string
-    {
+    protected function getConflictClause(
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): string {
         if ($mode === InsertMode::IGNORE) {
             return ' ON CONFLICT DO NOTHING';
         }
 
+        if ($mode === InsertMode::UPSERT && $updateColumns !== null && $conflictColumns !== null) {
+            $quotedConflictColumns = array_map($this->quoteColumnName(...), $conflictColumns);
+            $updates = [];
+            foreach ($updateColumns as $column) {
+                $quotedColumn = $this->quoteColumnName($column);
+                $updates[] = $quotedColumn . ' = EXCLUDED.' . $quotedColumn;
+            }
+
+            return ' ON CONFLICT (' . implode(', ', $quotedConflictColumns) . ') DO UPDATE SET ' . implode(', ', $updates);
+        }
+
         return '';
     }
 
diff --git a/src/Db/Adapter/SqliteAdapter.php b/src/Db/Adapter/SqliteAdapter.php
index 4c163755..0b49ded8 100644
--- a/src/Db/Adapter/SqliteAdapter.php
+++ b/src/Db/Adapter/SqliteAdapter.php
@@ -1702,4 +1702,28 @@ protected function getInsertPrefix(?InsertMode $mode = null): string
 
         return 'INSERT';
     }
+
+    /**
+     * Get the upsert clause for SQLite (ON CONFLICT ... DO UPDATE SET).
+     *
+     * @param \Migrations\Db\InsertMode|null $mode Insert mode
+     * @param array<string>|null $updateColumns Columns to update on conflict
+     * @param array<string>|null $conflictColumns Columns that define uniqueness for upsert
+     * @return string
+     */
+    protected function getUpsertClause(?InsertMode $mode, ?array $updateColumns, ?array $conflictColumns = null): string
+    {
+        if ($mode !== InsertMode::UPSERT || $updateColumns === null || $conflictColumns === null) {
+            return '';
+        }
+
+        $quotedConflictColumns = array_map($this->quoteColumnName(...), $conflictColumns);
+        $updates = [];
+        foreach ($updateColumns as $column) {
+            $quotedColumn = $this->quoteColumnName($column);
+            $updates[] = $quotedColumn . ' = excluded.' . $quotedColumn;
+        }
+
+        return ' ON CONFLICT (' . implode(', ', $quotedConflictColumns) . ') DO UPDATE SET ' . implode(', ', $updates);
+    }
 }
diff --git a/src/Db/Adapter/SqlserverAdapter.php b/src/Db/Adapter/SqlserverAdapter.php
index ec3c0fc0..71301bfe 100644
--- a/src/Db/Adapter/SqlserverAdapter.php
+++ b/src/Db/Adapter/SqlserverAdapter.php
@@ -1007,9 +1007,14 @@ public function migrated(MigrationInterface $migration, string $direction, strin
     /**
      * @inheritDoc
      */
-    public function insert(TableMetadata $table, array $row, ?InsertMode $mode = null): void
-    {
-        $sql = $this->generateInsertSql($table, $row, $mode);
+    public function insert(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
+        $sql = $this->generateInsertSql($table, $row, $mode, $updateColumns, $conflictColumns);
 
         $sql = $this->updateSQLForIdentityInsert($table->getName(), $sql);
 
@@ -1033,9 +1038,14 @@ public function insert(TableMetadata $table, array $row, ?InsertMode $mode = nul
     /**
      * @inheritDoc
      */
-    public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode = null): void
-    {
-        $sql = $this->generateBulkInsertSql($table, $rows, $mode);
+    public function bulkinsert(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
+        $sql = $this->generateBulkInsertSql($table, $rows, $mode, $updateColumns, $conflictColumns);
 
         $sql = $this->updateSQLForIdentityInsert($table->getName(), $sql);
 
diff --git a/src/Db/Adapter/TimedOutputAdapter.php b/src/Db/Adapter/TimedOutputAdapter.php
index b356403d..f5e11be6 100644
--- a/src/Db/Adapter/TimedOutputAdapter.php
+++ b/src/Db/Adapter/TimedOutputAdapter.php
@@ -84,22 +84,32 @@ function ($value) {
     /**
      * @inheritDoc
      */
-    public function insert(TableMetadata $table, array $row, ?InsertMode $mode = null): void
-    {
+    public function insert(
+        TableMetadata $table,
+        array $row,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
         $end = $this->startCommandTimer();
         $this->writeCommand('insert', [$table->getName()]);
-        parent::insert($table, $row, $mode);
+        parent::insert($table, $row, $mode, $updateColumns, $conflictColumns);
         $end();
     }
 
     /**
      * @inheritDoc
      */
-    public function bulkinsert(TableMetadata $table, array $rows, ?InsertMode $mode = null): void
-    {
+    public function bulkinsert(
+        TableMetadata $table,
+        array $rows,
+        ?InsertMode $mode = null,
+        ?array $updateColumns = null,
+        ?array $conflictColumns = null,
+    ): void {
         $end = $this->startCommandTimer();
         $this->writeCommand('bulkinsert', [$table->getName()]);
-        parent::bulkinsert($table, $rows, $mode);
+        parent::bulkinsert($table, $rows, $mode, $updateColumns, $conflictColumns);
         $end();
     }
 
diff --git a/src/Db/InsertMode.php b/src/Db/InsertMode.php
index 6a77eeff..e04b707c 100644
--- a/src/Db/InsertMode.php
+++ b/src/Db/InsertMode.php
@@ -28,4 +28,13 @@ enum InsertMode: string
      * - SQLite: INSERT OR IGNORE
      */
     case IGNORE = 'ignore';
+
+    /**
+     * UPSERT - inserts or updates rows on duplicate key conflicts
+     *
+     * - MySQL: ON DUPLICATE KEY UPDATE
+     * - PostgreSQL: ON CONFLICT (...) DO UPDATE SET
+     * - SQLite: ON CONFLICT (...) DO UPDATE SET
+     */
+    case UPSERT = 'upsert';
 }
diff --git a/src/Db/Table.php b/src/Db/Table.php
index 852d1ecd..6bf0021b 100644
--- a/src/Db/Table.php
+++ b/src/Db/Table.php
@@ -73,6 +73,20 @@ class Table
      */
     protected ?InsertMode $insertMode = null;
 
+    /**
+     * Columns to update on upsert conflict
+     *
+     * @var array<string>|null
+     */
+    protected ?array $upsertUpdateColumns = null;
+
+    /**
+     * Columns that define uniqueness for upsert conflict detection
+     *
+     * @var array<string>|null
+     */
+    protected ?array $upsertConflictColumns = null;
+
     /**
      * Primary key for this table.
      * Can either be a string or an array in case of composite
@@ -703,6 +717,34 @@ public function insertOrSkip(array $data)
         return $this->insert($data);
     }
 
+    /**
+     * Insert data into the table, updating specified columns on duplicate key conflicts.
+     *
+     * This method performs an "upsert" operation - inserting new rows and updating
+     * existing rows that conflict on the specified unique columns.
+     *
+     * Example:
+     * ```php
+     * $table->insertOrUpdate([
+     *     ['code' => 'USD', 'rate' => 1.0000],
+     *     ['code' => 'EUR', 'rate' => 0.9234],
+     * ], ['rate'], ['code']);
+     * ```
+     *
+     * @param array $data array of data in the same format as insert()
+     * @param array<string> $updateColumns Columns to update when a conflict occurs
+     * @param array<string> $conflictColumns Columns that define uniqueness (must have unique index)
+     * @return $this
+     */
+    public function insertOrUpdate(array $data, array $updateColumns, array $conflictColumns)
+    {
+        $this->insertMode = InsertMode::UPSERT;
+        $this->upsertUpdateColumns = $updateColumns;
+        $this->upsertConflictColumns = $conflictColumns;
+
+        return $this->insert($data);
+    }
+
     /**
      * Creates a table from the object instance.
      *
@@ -822,15 +864,29 @@ public function saveData(): void
         }
 
         if ($bulk) {
-            $this->getAdapter()->bulkinsert($this->table, $this->getData(), $this->insertMode);
+            $this->getAdapter()->bulkinsert(
+                $this->table,
+                $this->getData(),
+                $this->insertMode,
+                $this->upsertUpdateColumns,
+                $this->upsertConflictColumns,
+            );
         } else {
             foreach ($this->getData() as $row) {
-                $this->getAdapter()->insert($this->table, $row, $this->insertMode);
+                $this->getAdapter()->insert(
+                    $this->table,
+                    $row,
+                    $this->insertMode,
+                    $this->upsertUpdateColumns,
+                    $this->upsertConflictColumns,
+                );
             }
         }
 
         $this->resetData();
         $this->insertMode = null;
+        $this->upsertUpdateColumns = null;
+        $this->upsertConflictColumns = null;
     }
 
     /**
diff --git a/src/SeedInterface.php b/src/SeedInterface.php
index 6bc6a2b9..f566484f 100644
--- a/src/SeedInterface.php
+++ b/src/SeedInterface.php
@@ -155,6 +155,23 @@ public function insert(string $tableName, array $data): void;
      */
     public function insertOrSkip(string $tableName, array $data): void;
 
+    /**
+     * Insert data into a table, updating specified columns on duplicate key conflicts.
+     *
+     * This method performs an "upsert" operation - inserting new rows and updating
+     * existing rows that conflict on the specified unique columns.
+     *
+     * Uses ON DUPLICATE KEY UPDATE (MySQL), or ON CONFLICT ... DO UPDATE SET
+     * (PostgreSQL/SQLite).
+     *
+     * @param string $tableName Table name
+     * @param array $data Data
+     * @param array<string> $updateColumns Columns to update when a conflict occurs
+     * @param array<string> $conflictColumns Columns that define uniqueness (must have unique index)
+     * @return void
+     */
+    public function insertOrUpdate(string $tableName, array $data, array $updateColumns, array $conflictColumns): void;
+
     /**
      * Checks to see if a table exists.
      *
diff --git a/tests/TestCase/Db/Adapter/MysqlAdapterTest.php b/tests/TestCase/Db/Adapter/MysqlAdapterTest.php
index 2f5b1d65..083d505c 100644
--- a/tests/TestCase/Db/Adapter/MysqlAdapterTest.php
+++ b/tests/TestCase/Db/Adapter/MysqlAdapterTest.php
@@ -2973,4 +2973,82 @@ public function testAlgorithmWithMixedCase()
 
         $this->assertTrue($this->adapter->hasColumn('mixed_case', 'col2'));
     }
+
+    public function testInsertOrUpdateWithDuplicates()
+    {
+        $table = new Table('currencies', [], $this->adapter);
+        $table->addColumn('code', 'string', ['limit' => 3])
+            ->addColumn('rate', 'decimal', ['precision' => 10, 'scale' => 4])
+            ->addIndex('code', ['unique' => true])
+            ->create();
+
+        // First insert
+        $table->insertOrUpdate([
+            ['code' => 'USD', 'rate' => 1.0000],
+            ['code' => 'EUR', 'rate' => 0.9000],
+        ], ['rate'], ['code'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM currencies ORDER BY code');
+        $this->assertCount(2, $rows);
+        $this->assertEquals('0.9000', $rows[0]['rate']); // EUR
+        $this->assertEquals('1.0000', $rows[1]['rate']); // USD
+
+        // Update rates - should update existing rows
+        $table->insertOrUpdate([
+            ['code' => 'USD', 'rate' => 1.0500],
+            ['code' => 'EUR', 'rate' => 0.9234],
+            ['code' => 'GBP', 'rate' => 0.7800], // New row
+        ], ['rate'], ['code'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM currencies ORDER BY code');
+        $this->assertCount(3, $rows);
+        $this->assertEquals('0.9234', $rows[0]['rate']); // EUR updated
+        $this->assertEquals('0.7800', $rows[1]['rate']); // GBP new
+        $this->assertEquals('1.0500', $rows[2]['rate']); // USD updated
+    }
+
+    public function testInsertOrUpdateWithMultipleUpdateColumns()
+    {
+        $table = new Table('products', [], $this->adapter);
+        $table->addColumn('sku', 'string', ['limit' => 50])
+            ->addColumn('price', 'decimal', ['precision' => 10, 'scale' => 2])
+            ->addColumn('stock', 'integer')
+            ->addIndex('sku', ['unique' => true])
+            ->create();
+
+        // First insert
+        $table->insertOrUpdate([
+            ['sku' => 'ABC123', 'price' => 10.00, 'stock' => 100],
+        ], ['price', 'stock'], ['sku'])->save();
+
+        // Update both price and stock
+        $table->insertOrUpdate([
+            ['sku' => 'ABC123', 'price' => 15.00, 'stock' => 50],
+        ], ['price', 'stock'], ['sku'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM products');
+        $this->assertCount(1, $rows);
+        $this->assertEquals('15.00', $rows[0]['price']);
+        $this->assertEquals(50, $rows[0]['stock']);
+    }
+
+    public function testInsertOrUpdateModeResetsAfterSave()
+    {
+        $table = new Table('items', [], $this->adapter);
+        $table->addColumn('code', 'string', ['limit' => 10])
+            ->addColumn('name', 'string')
+            ->addIndex('code', ['unique' => true])
+            ->create();
+
+        // Use insertOrUpdate
+        $table->insertOrUpdate([
+            ['code' => 'ITEM1', 'name' => 'Item One'],
+        ], ['name'], ['code'])->save();
+
+        // Now use regular insert with duplicate - should throw exception
+        $this->expectException(PDOException::class);
+        $table->insert([
+            ['code' => 'ITEM1', 'name' => 'Different Name'],
+        ])->save();
+    }
 }
diff --git a/tests/TestCase/Db/Adapter/PostgresAdapterTest.php b/tests/TestCase/Db/Adapter/PostgresAdapterTest.php
index 56edf447..c832b84e 100644
--- a/tests/TestCase/Db/Adapter/PostgresAdapterTest.php
+++ b/tests/TestCase/Db/Adapter/PostgresAdapterTest.php
@@ -2905,4 +2905,82 @@ public function testInsertOrSkipWithoutDuplicates()
         $rows = $this->adapter->fetchAll('SELECT * FROM categories');
         $this->assertCount(2, $rows);
     }
+
+    public function testInsertOrUpdateWithDuplicates()
+    {
+        $table = new Table('currencies', [], $this->adapter);
+        $table->addColumn('code', 'string', ['limit' => 3])
+            ->addColumn('rate', 'decimal', ['precision' => 10, 'scale' => 4])
+            ->addIndex('code', ['unique' => true])
+            ->create();
+
+        // First insert
+        $table->insertOrUpdate([
+            ['code' => 'USD', 'rate' => 1.0000],
+            ['code' => 'EUR', 'rate' => 0.9000],
+        ], ['rate'], ['code'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM currencies ORDER BY code');
+        $this->assertCount(2, $rows);
+        $this->assertEquals('0.9000', $rows[0]['rate']); // EUR
+        $this->assertEquals('1.0000', $rows[1]['rate']); // USD
+
+        // Update rates - should update existing rows
+        $table->insertOrUpdate([
+            ['code' => 'USD', 'rate' => 1.0500],
+            ['code' => 'EUR', 'rate' => 0.9234],
+            ['code' => 'GBP', 'rate' => 0.7800], // New row
+        ], ['rate'], ['code'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM currencies ORDER BY code');
+        $this->assertCount(3, $rows);
+        $this->assertEquals('0.9234', $rows[0]['rate']); // EUR updated
+        $this->assertEquals('0.7800', $rows[1]['rate']); // GBP new
+        $this->assertEquals('1.0500', $rows[2]['rate']); // USD updated
+    }
+
+    public function testInsertOrUpdateWithMultipleUpdateColumns()
+    {
+        $table = new Table('products', [], $this->adapter);
+        $table->addColumn('sku', 'string', ['limit' => 50])
+            ->addColumn('price', 'decimal', ['precision' => 10, 'scale' => 2])
+            ->addColumn('stock', 'integer')
+            ->addIndex('sku', ['unique' => true])
+            ->create();
+
+        // First insert
+        $table->insertOrUpdate([
+            ['sku' => 'ABC123', 'price' => 10.00, 'stock' => 100],
+        ], ['price', 'stock'], ['sku'])->save();
+
+        // Update both price and stock
+        $table->insertOrUpdate([
+            ['sku' => 'ABC123', 'price' => 15.00, 'stock' => 50],
+        ], ['price', 'stock'], ['sku'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM products');
+        $this->assertCount(1, $rows);
+        $this->assertEquals('15.00', $rows[0]['price']);
+        $this->assertEquals(50, $rows[0]['stock']);
+    }
+
+    public function testInsertOrUpdateModeResetsAfterSave()
+    {
+        $table = new Table('items', [], $this->adapter);
+        $table->addColumn('code', 'string', ['limit' => 10])
+            ->addColumn('name', 'string')
+            ->addIndex('code', ['unique' => true])
+            ->create();
+
+        // Use insertOrUpdate
+        $table->insertOrUpdate([
+            ['code' => 'ITEM1', 'name' => 'Item One'],
+        ], ['name'], ['code'])->save();
+
+        // Now use regular insert with duplicate - should throw exception
+        $this->expectException(PDOException::class);
+        $table->insert([
+            ['code' => 'ITEM1', 'name' => 'Different Name'],
+        ])->save();
+    }
 }
diff --git a/tests/TestCase/Db/Adapter/SqliteAdapterTest.php b/tests/TestCase/Db/Adapter/SqliteAdapterTest.php
index 5c76b523..1fc925ae 100644
--- a/tests/TestCase/Db/Adapter/SqliteAdapterTest.php
+++ b/tests/TestCase/Db/Adapter/SqliteAdapterTest.php
@@ -3278,4 +3278,82 @@ public function testInsertOrSkipWithoutDuplicates()
         $rows = $this->adapter->fetchAll('SELECT * FROM categories');
         $this->assertCount(2, $rows);
     }
+
+    public function testInsertOrUpdateWithDuplicates()
+    {
+        $table = new Table('currencies', [], $this->adapter);
+        $table->addColumn('code', 'string', ['limit' => 3])
+            ->addColumn('rate', 'decimal', ['precision' => 10, 'scale' => 4])
+            ->addIndex('code', ['unique' => true])
+            ->create();
+
+        // First insert
+        $table->insertOrUpdate([
+            ['code' => 'USD', 'rate' => 1.0000],
+            ['code' => 'EUR', 'rate' => 0.9000],
+        ], ['rate'], ['code'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM currencies ORDER BY code');
+        $this->assertCount(2, $rows);
+        $this->assertEquals('0.9000', $rows[0]['rate']); // EUR
+        $this->assertEquals('1.0000', $rows[1]['rate']); // USD
+
+        // Update rates - should update existing rows
+        $table->insertOrUpdate([
+            ['code' => 'USD', 'rate' => 1.0500],
+            ['code' => 'EUR', 'rate' => 0.9234],
+            ['code' => 'GBP', 'rate' => 0.7800], // New row
+        ], ['rate'], ['code'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM currencies ORDER BY code');
+        $this->assertCount(3, $rows);
+        $this->assertEquals('0.9234', $rows[0]['rate']); // EUR updated
+        $this->assertEquals('0.7800', $rows[1]['rate']); // GBP new
+        $this->assertEquals('1.0500', $rows[2]['rate']); // USD updated
+    }
+
+    public function testInsertOrUpdateWithMultipleUpdateColumns()
+    {
+        $table = new Table('products', [], $this->adapter);
+        $table->addColumn('sku', 'string', ['limit' => 50])
+            ->addColumn('price', 'decimal', ['precision' => 10, 'scale' => 2])
+            ->addColumn('stock', 'integer')
+            ->addIndex('sku', ['unique' => true])
+            ->create();
+
+        // First insert
+        $table->insertOrUpdate([
+            ['sku' => 'ABC123', 'price' => 10.00, 'stock' => 100],
+        ], ['price', 'stock'], ['sku'])->save();
+
+        // Update both price and stock
+        $table->insertOrUpdate([
+            ['sku' => 'ABC123', 'price' => 15.00, 'stock' => 50],
+        ], ['price', 'stock'], ['sku'])->save();
+
+        $rows = $this->adapter->fetchAll('SELECT * FROM products');
+        $this->assertCount(1, $rows);
+        $this->assertEquals('15.00', $rows[0]['price']);
+        $this->assertEquals(50, $rows[0]['stock']);
+    }
+
+    public function testInsertOrUpdateModeResetsAfterSave()
+    {
+        $table = new Table('items', [], $this->adapter);
+        $table->addColumn('code', 'string', ['limit' => 10])
+            ->addColumn('name', 'string')
+            ->addIndex('code', ['unique' => true])
+            ->create();
+
+        // Use insertOrUpdate
+        $table->insertOrUpdate([
+            ['code' => 'ITEM1', 'name' => 'Item One'],
+        ], ['name'], ['code'])->save();
+
+        // Now use regular insert with duplicate - should throw exception
+        $this->expectException(PDOException::class);
+        $table->insert([
+            ['code' => 'ITEM1', 'name' => 'Different Name'],
+        ])->save();
+    }
 }