@@ -285,6 +285,12 @@ if [ "${PYTHON_MAJMIN_VERSION}" = "3.11" ]; then
     patch -p1 -i "${ROOT}/patch-tkinter-backport-tcl-9-311.patch"
 fi
 
+# Tcl 9 is always threaded and no longer exposes tcl_platform(threaded).
+# Backport https://github.com/python/cpython/pull/128103 (merged in 3.13+).
+if [ -n "${PYTHON_MEETS_MAXIMUM_VERSION_3_12}" ]; then
+    patch -p1 -i "${ROOT}/patch-tkinter-tcl9-thread-detection.patch"
+fi
+
 # BOLT instrumented binaries segfault in some test_embed tests for unknown reasons.
 # On 3.12 (minimum BOLT version), the segfault causes the test harness to
 # abort and BOLT optimization uses the partial test results. On 3.13, the segfault
 
@@ -0,0 +1,14 @@
+diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
+--- a/Modules/_tkinter.c
++++ b/Modules/_tkinter.c
+@@ -587,6 +587,10 @@ Tkapp_New(const char *screenName, const char *className,
+     v->interp = Tcl_CreateInterp();
+     v->wantobjects = wantobjects;
++#if TCL_MAJOR_VERSION >= 9
++    v->threaded = 1;
++#else
+     v->threaded = Tcl_GetVar2Ex(v->interp, "tcl_platform", "threaded",
+                                 TCL_GLOBAL_ONLY) != NULL;
++#endif
+     v->thread_id = Tcl_GetCurrentThread();
+     v->dispatching = 0;
@@ -326,6 +326,41 @@ def test_windows_tcltk(self):
             interpreter.eval("zlib decompress [zlib compress test]"), "test"
         )
 
+    @unittest.skipIf(
+        os.name == "nt" and "static" in os.environ["BUILD_OPTIONS"],
+        "Tcl/Tk is unavailable in Windows static builds",
+    )
+    def test_tcl_thread_dispatch(self):
+        # Tcl calls from another thread must wait for the interpreter's main loop.
+        # Use a subprocess so a broken dispatcher cannot crash or hang the suite.
+        probe = """
+import threading
+import tkinter
+
+interpreter = tkinter.Tcl()
+errors = []
+
+def worker():
+    try:
+        interpreter.call("set", "thread_dispatch_probe", "value")
+    except RuntimeError as exc:
+        errors.append(str(exc))
+
+thread = threading.Thread(target=worker, daemon=True)
+thread.start()
+thread.join(5)
+assert not thread.is_alive(), "Tcl call did not finish"
+assert errors == ["main thread is not in main loop"], errors
+"""
+        result = subprocess.run(
+            [sys.executable, "-c", probe],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            text=True,
+            timeout=10,
+        )
+        self.assertEqual(result.returncode, 0, result.stdout)
+
     @unittest.skipIf("TCL_LIBRARY" not in os.environ, "TCL_LIBRARY not set")
     @unittest.skipIf("DISPLAY" not in os.environ, "DISPLAY not set")
     def test_tkinter(self):