diff --git a/crates/oxc_parser/src/js/expression.rs b/crates/oxc_parser/src/js/expression.rs
index f1ab54714c1cb..880516a35dbd2 100644
--- a/crates/oxc_parser/src/js/expression.rs
+++ b/crates/oxc_parser/src/js/expression.rs
@@ -286,8 +286,12 @@ impl<'a> ParserImpl<'a> {
         let token = self.cur_token();
         let src = self.cur_src();
         let value = match token.kind {
-            Kind::Decimal | Kind::Binary | Kind::Octal | Kind::Hex => parse_int(src, token.kind),
-            Kind::Float | Kind::PositiveExponential | Kind::NegativeExponential => parse_float(src),
+            Kind::Decimal | Kind::Binary | Kind::Octal | Kind::Hex => {
+                parse_int(src, token.kind, token.has_separator())
+            }
+            Kind::Float | Kind::PositiveExponential | Kind::NegativeExponential => {
+                parse_float(src, token.has_separator())
+            }
             _ => unreachable!(),
         }
         .map_err(|err| diagnostics::invalid_number(err, token.span()))?;
diff --git a/crates/oxc_parser/src/lexer/number.rs b/crates/oxc_parser/src/lexer/number.rs
index c1e746325788c..438a278a8ee99 100644
--- a/crates/oxc_parser/src/lexer/number.rs
+++ b/crates/oxc_parser/src/lexer/number.rs
@@ -6,13 +6,35 @@ use std::borrow::Cow;
 
 use super::kind::Kind;
 
-// the string passed in has `_` removed from the lexer
-pub fn parse_int(s: &str, kind: Kind) -> Result<f64, &'static str> {
+pub fn parse_int(s: &str, kind: Kind, has_sep: bool) -> Result<f64, &'static str> {
+    let s = if has_sep { Cow::Owned(s.replace('_', "")) } else { Cow::Borrowed(s) };
+    let s = s.as_ref();
+    debug_assert!(!s.contains('_'));
+
+    // SAFETY: we just checked that `s` has no `_` characters
+    unsafe { parse_int_without_underscores_unchecked(s, kind) }
+}
+
+pub fn parse_float(s: &str, has_sep: bool) -> Result<f64, &'static str> {
+    let s = if has_sep { Cow::Owned(s.replace('_', "")) } else { Cow::Borrowed(s) };
+    debug_assert!(!s.contains('_'));
+
+    // SAFETY: we just checked that `s` has no `_` characters
+    unsafe { parse_float_without_underscores_unchecked(&s) }
+}
+
+/// # Safety
+///
+/// This function assumes that all `_` characters have been stripped from `s`.
+/// Violating this assumption does _not_ cause UB. However, this function is
+/// marked as unsafe to ensure consumers are aware of the assumption.
+unsafe fn parse_int_without_underscores_unchecked(
+    s: &str,
+    kind: Kind,
+) -> Result<f64, &'static str> {
     if kind == Kind::Decimal {
-        return parse_float(s);
+        return parse_float_without_underscores_unchecked(s);
     }
-    let s = if s.contains('_') { Cow::Owned(s.replace('_', "")) } else { Cow::Borrowed(s) };
-    let s = s.as_ref();
     match kind {
         Kind::Binary => Ok(parse_binary(&s[2..])),
         Kind::Octal => {
@@ -28,8 +50,12 @@ pub fn parse_int(s: &str, kind: Kind) -> Result<f64, &'static str> {
     }
 }
 
-pub fn parse_float(s: &str) -> Result<f64, &'static str> {
-    let s = if s.contains('_') { Cow::Owned(s.replace('_', "")) } else { Cow::Borrowed(s) };
+/// # Safety
+///
+/// This function assumes that all `_` characters have been stripped from `s`.
+/// Violating this assumption does _not_ cause UB. However, this function is
+/// marked as unsafe to ensure consumers are aware of the assumption.
+unsafe fn parse_float_without_underscores_unchecked(s: &str) -> Result<f64, &'static str> {
     s.parse::<f64>().map_err(|_| "invalid float")
 }
 
diff --git a/crates/oxc_parser/src/lexer/numeric.rs b/crates/oxc_parser/src/lexer/numeric.rs
index 963bc063e40e6..513d630bd2b70 100644
--- a/crates/oxc_parser/src/lexer/numeric.rs
+++ b/crates/oxc_parser/src/lexer/numeric.rs
@@ -53,6 +53,11 @@ impl<'a> Lexer<'a> {
             match c {
                 '_' => {
                     self.consume_char();
+                    // NOTE: it looks invalid numeric tokens are still parsed.
+                    // This seems to be a waste. It also requires us to put this
+                    // call here instead of after we ensure the next character
+                    // is a number character
+                    self.token.set_has_separator();
                     if self.peek().is_some_and(|c| kind.matches_number_char(c)) {
                         self.consume_char();
                     } else {
@@ -134,6 +139,11 @@ impl<'a> Lexer<'a> {
             match c {
                 '_' => {
                     self.consume_char();
+                    // NOTE: it looks invalid numeric tokens are still parsed.
+                    // This seems to be a waste. It also requires us to put this
+                    // call here instead of after we ensure the next character
+                    // is an ASCII digit
+                    self.token.set_has_separator();
                     if self.peek().is_some_and(|c| c.is_ascii_digit()) {
                         self.consume_char();
                     } else {
diff --git a/crates/oxc_parser/src/lexer/token.rs b/crates/oxc_parser/src/lexer/token.rs
index 84119516e5939..9b92e5591e422 100644
--- a/crates/oxc_parser/src/lexer/token.rs
+++ b/crates/oxc_parser/src/lexer/token.rs
@@ -26,10 +26,16 @@ pub struct Token {
     /// [Lexer::escaped_templates]: [super::Lexer::escaped_templates]
     pub escaped: bool,
 
+    /// True if for numeric literal tokens that contain separator characters (`_`).
+    ///
+    /// Numeric literals are defined in Section 12.9.3 of the ECMAScript
+    /// standard and include [`Kind::Decimal`], [`Kind::Binary`],
+    /// [`Kind::Octal`], [`Kind::Hex`], etc.
+    has_separator: bool,
+
     // Padding to fill to 16 bytes.
     // This makes copying a `Token` 1 x xmmword load & store, rather than 1 x dword + 1 x qword
     // and `Token::default()` is 1 x xmmword store, rather than 1 x dword + 1 x qword.
-    _padding1: u8,
     _padding2: u32,
 }
 
@@ -50,4 +56,15 @@ impl Token {
     pub fn escaped(&self) -> bool {
         self.escaped
     }
+
+    #[inline]
+    pub fn has_separator(&self) -> bool {
+        debug_assert!(!self.has_separator || self.kind.is_number());
+        self.has_separator
+    }
+
+    pub(crate) fn set_has_separator(&mut self) {
+        debug_assert!(!self.has_separator || self.kind.is_number() || self.kind == Kind::default());
+        self.has_separator = true;
+    }
 }