diff --git a/graalpython/com.oracle.graal.python.test/src/tests/test_sys.py b/graalpython/com.oracle.graal.python.test/src/tests/test_sys.py
index 31f9e974aa..3ecd5e9d16 100644
--- a/graalpython/com.oracle.graal.python.test/src/tests/test_sys.py
+++ b/graalpython/com.oracle.graal.python.test/src/tests/test_sys.py
@@ -133,3 +133,9 @@ def test_getwindowsversion_java_backend(self):
                     capture_output=True, text=True, check=True,
                 )
                 self.assertEqual(result.stdout.splitlines(), [str(expected), str(expected)])
+
+
+class SysFlagsTests(unittest.TestCase):
+    def test_debug_is_zero_by_default(self):
+        # CPython sets sys.flags.debug only when -d is given, not from -O.
+        self.assertEqual(sys.flags.debug, 0)
diff --git a/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java b/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java
index e76f46c806..ba1465c999 100644
--- a/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java
+++ b/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java
@@ -694,7 +694,7 @@ public void postInitialize0(Python3Core core) {
         PList sysPaths = PFactory.createList(language, path);
         sys.setAttribute(tsInternedLiteral("path"), sysPaths);
         sys.setAttribute(tsInternedLiteral("flags"), PFactory.createStructSeq(language, SysModuleBuiltins.FLAGS_DESC,
-                        PInt.intValue(!language.getEngineOption(PythonOptions.PythonOptimizeFlag)), // debug
+                        0, // debug
                         PInt.intValue(context.getOption(PythonOptions.InspectFlag)), // inspect
                         PInt.intValue(context.getOption(PythonOptions.TerminalIsInteractive)), // interactive
                         PInt.intValue(language.getEngineOption(PythonOptions.PythonOptimizeFlag)), // optimize