diff --git a/Lib/idlelib/idle_test/test_iomenu.py b/Lib/idlelib/idle_test/test_iomenu.py
index 90a23620354f313..3ef9fcbc5bb569b 100644
--- a/Lib/idlelib/idle_test/test_iomenu.py
+++ b/Lib/idlelib/idle_test/test_iomenu.py
@@ -1,11 +1,13 @@
 "Test , coverage 17%."
 
 from idlelib import iomenu
+import os
 import unittest
+from unittest import mock
 from test.support import requires
-from tkinter import Tk
+from tkinter import Tk, TclVersion
 from idlelib.editor import EditorWindow
-from idlelib import util
+from idlelib import macosx, util
 from idlelib.idle_test.mock_idle import Func
 
 # Fail if either tokenize.open and t.detect_encoding does not exist.
@@ -38,6 +40,17 @@ def tearDownClass(cls):
     def test_init(self):
         self.assertIs(self.io.editwin, self.editwin)
 
+    def test_defaultfilename_tilde(self):
+        # gh-59568: escape a leading tilde for Tcl older than 9.
+        io = self.io
+        with mock.patch.object(io, 'filename', os.path.join('dir', '~file.py')):
+            dirname, base = io.defaultfilename()
+        self.assertEqual(dirname, 'dir')
+        if TclVersion < 9 and not macosx.isAquaTk():
+            self.assertEqual(base, os.path.join(os.curdir, '~file.py'))
+        else:
+            self.assertEqual(base, '~file.py')
+
     def test_fixnewlines_end(self):
         eq = self.assertEqual
         io = self.io
diff --git a/Lib/idlelib/iomenu.py b/Lib/idlelib/iomenu.py
index 9560aaa2da0c424..96bf6078f12c3ec 100644
--- a/Lib/idlelib/iomenu.py
+++ b/Lib/idlelib/iomenu.py
@@ -5,11 +5,13 @@
 import tempfile
 import tokenize
 
+from tkinter import TclVersion
 from tkinter import filedialog
 from tkinter import messagebox
 from tkinter.simpledialog import askstring  # loadfile encoding.
 
 from idlelib.config import idleConf
+from idlelib import macosx
 from idlelib.util import py_extensions
 
 py_extensions = ' '.join("*"+ext for ext in py_extensions)
@@ -405,7 +407,12 @@ def askopenfile(self):
 
     def defaultfilename(self, mode="open"):
         if self.filename:
-            return os.path.split(self.filename)
+            dirname, base = os.path.split(self.filename)
+            if base[:1] == '~' and TclVersion < 9 and not macosx.isAquaTk():
+                # Tcl before version 9 substitutes a leading tilde in
+                # a file name (gh-59568).  The macOS dialog does not.
+                base = os.path.join(os.curdir, base)
+            return dirname, base
         elif self.dirname:
             return self.dirname, ""
         else:
diff --git a/Misc/NEWS.d/next/IDLE/2026-09-16-18-00-00.gh-issue-59568.tilde1.rst b/Misc/NEWS.d/next/IDLE/2026-09-16-18-00-00.gh-issue-59568.tilde1.rst
new file mode 100644
index 000000000000000..3eabd4e176ad8a6
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2026-09-16-18-00-00.gh-issue-59568.tilde1.rst
@@ -0,0 +1,2 @@
+Fix "Save As" and "Open" dialogs in IDLE for a file whose name starts with
+``~`` on Windows and X11 with Tcl/Tk older than 9.