diff --git a/src/wp-includes/cache-compat.php b/src/wp-includes/cache-compat.php
index 98f489931e..a9408ca611 100644
--- a/src/wp-includes/cache-compat.php
+++ b/src/wp-includes/cache-compat.php
@@ -123,3 +123,21 @@ if ( ! function_exists( 'wp_cache_delete_multiple' ) ) :
 		return $values;
 	}
 endif;
+
+if ( ! function_exists( 'wp_cache_flush_runtime' ) ) :
+	/**
+	 * Removes all cache items from the in-memory runtime cache.
+	 *
+	 * Compat function to mimic wp_cache_flush_runtime().
+	 *
+	 * @ignore
+	 * @since 6.0.0
+	 *
+	 * @see wp_cache_flush_runtime()
+	 *
+	 * @return bool True on success, false on failure.
+	 */
+	function wp_cache_flush_runtime() {
+		return wp_using_ext_object_cache() ? false : wp_cache_flush();
+	}
+endif;
diff --git a/src/wp-includes/cache.php b/src/wp-includes/cache.php
index b7e88efbd0..2a2a98f6d3 100644
--- a/src/wp-includes/cache.php
+++ b/src/wp-includes/cache.php
@@ -268,6 +268,19 @@ function wp_cache_flush() {
 	return $wp_object_cache->flush();
 }
 
+/**
+ * Removes all cache items from the in-memory runtime cache.
+ *
+ * @since 6.0.0
+ *
+ * @see WP_Object_Cache::flush()
+ *
+ * @return bool True on success, false on failure.
+ */
+function wp_cache_flush_runtime() {
+	return wp_cache_flush();
+}
+
 /**
  * Closes the cache.
  *
diff --git a/tests/phpunit/tests/pluggable.php b/tests/phpunit/tests/pluggable.php
index a1dbda3506..c7b0225987 100644
--- a/tests/phpunit/tests/pluggable.php
+++ b/tests/phpunit/tests/pluggable.php
@@ -326,6 +326,7 @@ class Tests_Pluggable extends WP_UnitTestCase {
 						'group'  => '',
 					),
 					'wp_cache_flush'                     => array(),
+					'wp_cache_flush_runtime'             => array(),
 					'wp_cache_close'                     => array(),
 					'wp_cache_add_global_groups'         => array( 'groups' ),
 					'wp_cache_add_non_persistent_groups' => array( 'groups' ),