diff --git a/Lib/idlelib/idle_test/test_util.py b/Lib/idlelib/idle_test/test_util.py
index d90a0784075d3e0..7e69a5c972f1f69 100644
--- a/Lib/idlelib/idle_test/test_util.py
+++ b/Lib/idlelib/idle_test/test_util.py
@@ -8,6 +8,7 @@
 import tkinter
 from tkinter import EventType
 from idlelib import util
+from idlelib.multicall import MC_SHIFT, MC_CONTROL
 from idlelib.idle_test.mock_tk import Event
 
 
@@ -72,9 +73,9 @@ class WheelEventTest(unittest.TestCase):
 
     # An unmapped widget has no height and does not scroll by lines,
     # so record the yview call instead of a real scroll.
-    def event(self, event_type, delta=0, num='??'):
+    def event(self, event_type, delta=0, num='??', state=0):
         # Tk leaves num '??' for a wheel event and delta 0 for a button.
-        return Event(type=event_type, delta=delta, num=num,
+        return Event(type=event_type, delta=delta, num=num, state=state,
                      widget=mock.Mock())
 
     def scroll(self, event, widget=None):
@@ -82,8 +83,17 @@ def scroll(self, event, widget=None):
         self.assertEqual(util.wheel_event(event, widget), 'break')
         scrolled = event.widget if widget is None else widget
         scrolled.yview.assert_called_once()
+        scrolled.xview.assert_not_called()
         return scrolled.yview.call_args.args
 
+    def hscroll(self, event, widget=None):
+        "Return the arguments of the xview call."
+        self.assertEqual(util.wheel_event(event, widget), 'break')
+        scrolled = event.widget if widget is None else widget
+        scrolled.xview.assert_called_once()
+        scrolled.yview.assert_not_called()
+        return scrolled.xview.call_args.args
+
     def test_mousewheel(self):
         # Delta is positive for up on all systems.
         for delta in 120, 1, 1200:
@@ -100,6 +110,26 @@ def test_buttons(self):
         self.assertEqual(self.scroll(self.event(EventType.ButtonPress, num=5)),
                          ('scroll', 5, 'units'))
 
+    def test_horizontal(self):
+        # The Shift modifier means horizontal.
+        for delta in 120, 1, 1200:
+            self.assertEqual(self.hscroll(self.event(EventType.MouseWheel,
+                                                     delta, state=MC_SHIFT)),
+                             ('scroll', -5, 'units'))
+            self.assertEqual(self.hscroll(self.event(EventType.MouseWheel,
+                                                     -delta, state=MC_SHIFT)),
+                             ('scroll', 5, 'units'))
+        self.assertEqual(self.hscroll(self.event(EventType.ButtonPress,
+                                                 num=4, state=MC_SHIFT)),
+                         ('scroll', -5, 'units'))
+        self.assertEqual(self.hscroll(self.event(EventType.ButtonPress,
+                                                 num=5, state=MC_SHIFT)),
+                         ('scroll', 5, 'units'))
+        # Other modifiers alone do not.
+        self.assertEqual(self.scroll(self.event(EventType.MouseWheel,
+                                                120, state=MC_CONTROL)),
+                         ('scroll', -5, 'units'))
+
     def test_widget_argument(self):
         # A tree label scrolls the canvas, not itself.
         event = self.event(EventType.MouseWheel, 120)
diff --git a/Lib/idlelib/util.py b/Lib/idlelib/util.py
index f408daf728d4a97..f80bd5078d667a3 100644
--- a/Lib/idlelib/util.py
+++ b/Lib/idlelib/util.py
@@ -14,6 +14,8 @@
 """
 import sys
 
+from idlelib.multicall import MC_SHIFT
+
 # .pyw is for Windows; .pyi is for typing stub files.
 # The extension order is needed for iomenu open/save dialogs.
 py_extensions = ('.py', '.pyw', '.pyi')
@@ -101,16 +103,24 @@ def wheel_event(event, widget=None):
     A <Button-4> or <Button-5> event (see x11_buttons) says up or down
     by its number, and has no delta; a wheel event has no number.
 
+    If the Shift modifier is set in event.state, scroll horizontally.
+    Tk 9 reports horizontal scrolling, as with a touchpad, as a
+    <Shift-MouseWheel> event on all systems, and Tk 8.6.10 and newer
+    on X11 report it as a <Shift-Button-4> or <Shift-Button-5> event.
+
     The widget parameter is needed so tree label bindings can pass the
     underlying canvas.  If tree is replaced by ttk.Treeview, it can go.
 
-    This function depends on widget.yview to not be overridden by
-    a subclass.
+    This function depends on widget.xview and widget.yview to not be
+    overridden by a subclass.
     """
     up = event.num == 4 if event.num in (4, 5) else event.delta > 0
     lines = -5 if up else 5
     widget = event.widget if widget is None else widget
-    widget.yview('scroll', lines, 'units')
+    if event.state & MC_SHIFT:
+        widget.xview('scroll', lines, 'units')
+    else:
+        widget.yview('scroll', lines, 'units')
     return 'break'
 
 
diff --git a/Misc/ACKS b/Misc/ACKS
index 4a3f6482294e087..923c68ef23ff8f5 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1975,6 +1975,7 @@ John Tromp
 Diane Trout
 Jason Trowbridge
 Steven Troxler
+Daniel Tsvetkov
 Brent Tubbs
 Anthony Tuininga
 Erno Tukia
diff --git a/Misc/NEWS.d/next/IDLE/2025-10-20-21-40-38.gh-issue-140389.vOs4uy.rst b/Misc/NEWS.d/next/IDLE/2025-10-20-21-40-38.gh-issue-140389.vOs4uy.rst
new file mode 100644
index 000000000000000..2e48176610b81d1
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2025-10-20-21-40-38.gh-issue-140389.vOs4uy.rst
@@ -0,0 +1 @@
+Fixed trackpad scrolling direction in :mod:`IDLE <idlelib>`