diff --git a/src/wp-includes/rest-api.php b/src/wp-includes/rest-api.php
index 5548ecf5c6f45..dc8a8d938eac4 100644
--- a/src/wp-includes/rest-api.php
+++ b/src/wp-includes/rest-api.php
@@ -1569,8 +1569,22 @@ function rest_is_boolean( $maybe_bool ) {
  * @param mixed $maybe_integer The value being evaluated.
  * @return bool True if an integer, otherwise false.
  */
-function rest_is_integer( $maybe_integer ) {
-	return is_numeric( $maybe_integer ) && round( (float) $maybe_integer ) === (float) $maybe_integer;
+function rest_is_integer( $maybe_integer ): bool {
+	if ( is_int( $maybe_integer ) ) {
+		return true;
+	}
+
+	// A canonical integer string of any magnitude — verified without float conversion.
+	if ( is_string( $maybe_integer ) && preg_match( '/^\s*[+-]?[0-9]+\s*$/', $maybe_integer ) ) {
+		return true;
+	}
+
+	// Decimal and scientific-notation strings (and floats) keep their historical behavior.
+	if ( ! is_numeric( $maybe_integer ) ) {
+		return false;
+	}
+	$float_value = (float) $maybe_integer;
+	return floor( $float_value ) === $float_value;
 }
 
 /**
diff --git a/tests/phpunit/tests/rest-api.php b/tests/phpunit/tests/rest-api.php
index 90de3e13eecea..cfc53bd434114 100644
--- a/tests/phpunit/tests/rest-api.php
+++ b/tests/phpunit/tests/rest-api.php
@@ -2291,6 +2291,7 @@ public function data_rest_sanitize_array() {
 
 	/**
 	 * @ticket 51146
+	 * @ticket 65271
 	 *
 	 * @dataProvider data_rest_is_integer
 	 *
@@ -2307,7 +2308,12 @@ public function test_rest_is_integer( $expected, $value ) {
 		}
 	}
 
-	public function data_rest_is_integer() {
+	/**
+	 * Data provider for {@see self::test_rest_is_integer()}.
+	 *
+	 * @return list<array{ bool, mixed }>
+	 */
+	public function data_rest_is_integer(): array {
 		return array(
 			array(
 				true,
@@ -2341,6 +2347,14 @@ public function data_rest_is_integer() {
 				false,
 				'5.5',
 			),
+			array(
+				false,
+				-5.5,
+			),
+			array(
+				false,
+				'-5.5',
+			),
 			array(
 				false,
 				array(),
@@ -2349,6 +2363,40 @@ public function data_rest_is_integer() {
 				false,
 				true,
 			),
+			array(
+				true,
+				'15e0',
+			),
+			array(
+				true,
+				'15e+0',
+			),
+			array(
+				true,
+				'15e-0',
+			),
+			array(
+				false,
+				'15e-1',
+			),
+
+			// The following values failed with PHP 8.4 when rest_is_integer() used round() in its implementation.
+			array(
+				true,
+				2 ** 52,
+			),
+			array(
+				true,
+				2 ** 52 + 2,
+			),
+			array(
+				true,
+				'4503599627370496',  // 2 ** 52
+			),
+			array(
+				true,
+				'4503599627370498',  // 2 ** 52 + 2
+			),
 		);
 	}