diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ae93d03a..ee799ca0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -98,8 +98,8 @@ jobs:
       - name: Confirm bench works
         run: dotnet run --project tests/Temporalio.SimpleBench/Temporalio.SimpleBench.csproj -- --workflow-count 5 --max-cached-workflows 100 --max-concurrent 100
 
-      - name: Test cloud
-        # Only supported in non-fork runs, since secrets are not available in forks
+      - name: Test cloud (mTLS)
+        # Only supported in non-fork runs, since secrets are not available in forks.
         if: ${{ matrix.cloudTestTarget && (github.event.pull_request.head.repo.full_name == '' || github.event.pull_request.head.repo.full_name == 'temporalio/sdk-dotnet') }}
         env:
           TEMPORAL_TEST_CLIENT_TARGET_HOST: ${{ vars.TEMPORAL_CLIENT_NAMESPACE }}.tmprl.cloud:7233
@@ -108,6 +108,16 @@ jobs:
           TEMPORAL_TEST_CLIENT_KEY: ${{ secrets.TEMPORAL_CLIENT_KEY }}
         run: dotnet run --project tests/Temporalio.Tests -- -verbose -method "*.ExecuteWorkflowAsync_Simple_Succeeds"
 
+      - name: Test cloud (API key)
+        # Only supported in non-fork runs, since secrets are not available in forks.
+        # Uses API key auth to test auto-TLS feature (TLS auto-enabled when API key provided).
+        if: ${{ matrix.cloudTestTarget && (github.event.pull_request.head.repo.full_name == '' || github.event.pull_request.head.repo.full_name == 'temporalio/sdk-dotnet') }}
+        env:
+          TEMPORAL_TEST_CLIENT_TARGET_HOST: us-west-2.aws.api.temporal.io:7233
+          TEMPORAL_TEST_CLIENT_NAMESPACE: ${{ vars.TEMPORAL_CLIENT_NAMESPACE }}
+          TEMPORAL_CLIENT_CLOUD_API_KEY: ${{ secrets.TEMPORAL_CLIENT_CLOUD_API_KEY }}
+        run: dotnet run --project tests/Temporalio.Tests -- -verbose -method "*.ExecuteWorkflowAsync_Simple_Succeeds"
+
       - name: Test cloud operations client
         # Only supported in non-fork runs, since secrets are not available in forks
         if: ${{ matrix.cloudTestTarget && (github.event.pull_request.head.repo.full_name == '' || github.event.pull_request.head.repo.full_name == 'temporalio/sdk-dotnet') }}
diff --git a/src/Temporalio/Bridge/OptionsExtensions.cs b/src/Temporalio/Bridge/OptionsExtensions.cs
index 2ad8cd19..3b9214eb 100644
--- a/src/Temporalio/Bridge/OptionsExtensions.cs
+++ b/src/Temporalio/Bridge/OptionsExtensions.cs
@@ -241,7 +241,19 @@ public static unsafe Interop.TemporalCoreClientOptions ToInteropOptions(
             {
                 throw new ArgumentException("Identity missing from options.");
             }
-            var scheme = options.Tls == null ? "http" : "https";
+
+            // Auto-enable TLS when API key is provided and TLS is not explicitly disabled
+            var tls = options.Tls;
+            if (tls?.Disabled == true)
+            {
+                tls = null;
+            }
+            else if (tls == null && !string.IsNullOrEmpty(options.ApiKey))
+            {
+                tls = new Temporalio.Client.TlsOptions();
+            }
+
+            var scheme = tls == null ? "http" : "https";
             return new Interop.TemporalCoreClientOptions()
             {
                 target_url = scope.ByteArray($"{scheme}://{options.TargetHost}"),
@@ -251,7 +263,7 @@ public static unsafe Interop.TemporalCoreClientOptions ToInteropOptions(
                 api_key = scope.ByteArray(options.ApiKey),
                 identity = scope.ByteArray(options.Identity),
                 tls_options =
-                    options.Tls == null ? null : scope.Pointer(options.Tls.ToInteropOptions(scope)),
+                    tls == null ? null : scope.Pointer(tls.ToInteropOptions(scope)),
                 retry_options =
                     options.RpcRetry == null
                         ? null
diff --git a/src/Temporalio/Client/TemporalConnectionOptions.cs b/src/Temporalio/Client/TemporalConnectionOptions.cs
index 5ab5016f..733ec85f 100644
--- a/src/Temporalio/Client/TemporalConnectionOptions.cs
+++ b/src/Temporalio/Client/TemporalConnectionOptions.cs
@@ -38,7 +38,9 @@ public TemporalConnectionOptions()
         /// Gets or sets the TLS options for connection.
         /// </summary>
         /// <remarks>
-        /// This must be set, even to a default instance, to do any TLS connection.
+        /// This must be set, even to a default instance, to do any TLS connection. If not set and
+        /// <see cref="ApiKey"/> is provided, TLS will be automatically enabled with default options.
+        /// To explicitly disable TLS, set instance with <see cref="TlsOptions.Disabled"/> set to true.
         /// </remarks>
         public TlsOptions? Tls { get; set; }
 
diff --git a/src/Temporalio/Client/TlsOptions.cs b/src/Temporalio/Client/TlsOptions.cs
index 9cc1caf8..38565a65 100644
--- a/src/Temporalio/Client/TlsOptions.cs
+++ b/src/Temporalio/Client/TlsOptions.cs
@@ -34,6 +34,15 @@ public class TlsOptions : ICloneable
         /// </remarks>
         public byte[]? ClientPrivateKey { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether TLS should be explicitly disabled.
+        /// </summary>
+        /// <remarks>
+        /// When set to true, TLS will be disabled even when an API key is provided (which normally
+        /// auto-enables TLS).
+        /// </remarks>
+        public bool Disabled { get; set; }
+
         /// <summary>
         /// Create a shallow copy of these options.
         /// </summary>
diff --git a/tests/Temporalio.Tests/Client/TemporalConnectionOptionsTests.cs b/tests/Temporalio.Tests/Client/TemporalConnectionOptionsTests.cs
new file mode 100644
index 00000000..e5314b64
--- /dev/null
+++ b/tests/Temporalio.Tests/Client/TemporalConnectionOptionsTests.cs
@@ -0,0 +1,72 @@
+namespace Temporalio.Tests.Client;
+
+using Temporalio.Bridge;
+using Temporalio.Client;
+using Xunit;
+
+public unsafe class TemporalConnectionOptionsTests
+{
+    [Fact]
+    public void ToInteropOptions_AutoEnablesTls_WhenApiKeyProvidedAndTlsNotSet()
+    {
+        var options = new TemporalConnectionOptions("localhost:7233")
+        {
+            ApiKey = "test-api-key",
+            Identity = "test-identity",
+        };
+
+        using var scope = new Scope();
+        var interopOptions = options.ToInteropOptions(scope);
+
+        // TLS should be auto-enabled when API key is provided and TLS not set
+        Assert.True(interopOptions.tls_options != null);
+    }
+
+    [Fact]
+    public void ToInteropOptions_TlsDisabled_WhenExplicitlyDisabledWithApiKey()
+    {
+        var options = new TemporalConnectionOptions("localhost:7233")
+        {
+            ApiKey = "test-api-key",
+            Identity = "test-identity",
+            Tls = new TlsOptions { Disabled = true },
+        };
+
+        using var scope = new Scope();
+        var interopOptions = options.ToInteropOptions(scope);
+
+        // TLS should remain disabled when explicitly disabled, even with API key
+        Assert.True(interopOptions.tls_options == null);
+    }
+
+    [Fact]
+    public void ToInteropOptions_TlsDisabled_WhenNoApiKeyAndTlsNotSet()
+    {
+        var options = new TemporalConnectionOptions("localhost:7233")
+        {
+            Identity = "test-identity",
+        };
+
+        using var scope = new Scope();
+        var interopOptions = options.ToInteropOptions(scope);
+
+        // TLS should be disabled when no API key and TLS not set
+        Assert.True(interopOptions.tls_options == null);
+    }
+
+    [Fact]
+    public void ToInteropOptions_TlsEnabled_WhenExplicitlySet()
+    {
+        var options = new TemporalConnectionOptions("localhost:7233")
+        {
+            Identity = "test-identity",
+            Tls = new TlsOptions(),
+        };
+
+        using var scope = new Scope();
+        var interopOptions = options.ToInteropOptions(scope);
+
+        // TLS should be enabled when explicitly set
+        Assert.True(interopOptions.tls_options != null);
+    }
+}
diff --git a/tests/Temporalio.Tests/Temporalio.Tests.csproj b/tests/Temporalio.Tests/Temporalio.Tests.csproj
index b76421cd..96534999 100644
--- a/tests/Temporalio.Tests/Temporalio.Tests.csproj
+++ b/tests/Temporalio.Tests/Temporalio.Tests.csproj
@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
     <GenerateProgramFile>false</GenerateProgramFile>
     <ImplicitUsings>enable</ImplicitUsings>
diff --git a/tests/Temporalio.Tests/WorkflowEnvironment.cs b/tests/Temporalio.Tests/WorkflowEnvironment.cs
index 82dced9a..43653835 100644
--- a/tests/Temporalio.Tests/WorkflowEnvironment.cs
+++ b/tests/Temporalio.Tests/WorkflowEnvironment.cs
@@ -40,7 +40,8 @@ public async Task InitializeAsync()
             };
             var clientCert = Environment.GetEnvironmentVariable("TEMPORAL_TEST_CLIENT_CERT");
             var clientKey = Environment.GetEnvironmentVariable("TEMPORAL_TEST_CLIENT_KEY");
-            if ((clientCert == null) != (clientKey == null))
+            var apiKey = Environment.GetEnvironmentVariable("TEMPORAL_CLIENT_CLOUD_API_KEY");
+            if (clientCert == null != (clientKey == null))
             {
                 throw new InvalidOperationException("Must have both cert/key or neither");
             }
@@ -52,6 +53,11 @@ public async Task InitializeAsync()
                     ClientPrivateKey = System.Text.Encoding.ASCII.GetBytes(clientKey),
                 };
             }
+            if (apiKey != null)
+            {
+                // API key auto-enables TLS when Tls is not set
+                options.ApiKey = apiKey;
+            }
             env = new(await TemporalClient.ConnectAsync(options));
         }
         else