diff --git a/Lib/idlelib/editor.py b/Lib/idlelib/editor.py
index 855d375055653a..b7b8b7cc7b5bda 100644
--- a/Lib/idlelib/editor.py
+++ b/Lib/idlelib/editor.py
@@ -921,8 +921,8 @@ def get_saved(self):
     def set_saved(self, flag):
         self.undo.set_saved(flag)
 
-    def reset_undo(self):
-        self.undo.reset_undo()
+    def reset_undo(self, set_saved_flag=True):
+        self.undo.reset_undo(set_saved_flag)
 
     def short_title(self):
         filename = self.io.filename
diff --git a/Lib/idlelib/idle_test/test_undo.py b/Lib/idlelib/idle_test/test_undo.py
index e872927a6c6d99..2699d014885a04 100644
--- a/Lib/idlelib/idle_test/test_undo.py
+++ b/Lib/idlelib/idle_test/test_undo.py
@@ -87,6 +87,38 @@ def test_dump_event(self):
         text.event_generate('<<undo>>')
         self.assertTupleEqual((d.pointer, d.can_merge), (2, False))
 
+    def test_reset_undo(self):
+        eq = self.assertEqual
+        d = self.delegator
+        orig_sch = d.saved_change_hook
+        d.saved_change_hook = Mock()
+
+        def set_text():
+            self.text.insert('insert', 'spam')
+            self.text.insert('insert', '\n')
+            self.text.insert('insert', 'this is the second line')
+
+        set_text()
+        eq(d.pointer, len(d.undolist))
+        self.assertNotEqual(d.pointer, d.saved)
+
+        # Default to set_saved_flag to True.
+        d.reset_undo()
+        eq(d.pointer, 0)
+        eq(d.undolist, [])
+        eq(d.undoblock, 0)
+        eq(d.saved, d.pointer)
+
+        # Don't mark as saved upon reset.
+        set_text()
+        d.reset_undo(False)
+        eq(d.pointer, 0)
+        eq(d.undolist, [])
+        eq(d.undoblock, 0)
+        eq(d.saved, -1)
+
+        d.saved_change_hook = orig_sch
+
     def test_get_set_saved(self):
         # test the getter method get_saved
         # test the setter method set_saved
diff --git a/Lib/idlelib/iomenu.py b/Lib/idlelib/iomenu.py
index 3414c7b3aff4b3..2d738aa66376b4 100644
--- a/Lib/idlelib/iomenu.py
+++ b/Lib/idlelib/iomenu.py
@@ -144,8 +144,8 @@ def get_saved(self):
     def set_saved(self, flag):
         self.editwin.set_saved(flag)
 
-    def reset_undo(self):
-        self.editwin.reset_undo()
+    def reset_undo(self, set_saved_flag=True):
+        self.editwin.reset_undo(set_saved_flag)
 
     filename_change_hook = None
 
diff --git a/Lib/idlelib/pyshell.py b/Lib/idlelib/pyshell.py
index 168eeae9ad8e9c..d9eeeb61daea37 100755
--- a/Lib/idlelib/pyshell.py
+++ b/Lib/idlelib/pyshell.py
@@ -1247,6 +1247,18 @@ def restart_shell(self, event=None):
         self.interp.restart_subprocess(with_cwd=True)
 
     def showprompt(self):
+        """Display the prompt in the shell window.
+
+        Before the prompt is displayed, the text from the prior
+        prompt is stored in history and the text marks are moved
+        ahead to end of the text.  Once the prompt is written, the
+        text mark is set to capture insertions and the line and
+        column within the status bar is updated.  Also, the buffer
+        for undo and redo is cleared as the shell only allows
+        that functionality on the current line.  reset_undo is
+        called with a saved_flag of False so that clearing the
+        undo doesn't also get marked as a file save.
+        """
         self.resetoutput()
         try:
             s = str(sys.ps1)
@@ -1255,7 +1267,7 @@ def showprompt(self):
         self.console.write(s)
         self.text.mark_set("insert", "end-1c")
         self.set_line_and_column()
-        self.io.reset_undo()
+        self.io.reset_undo(set_saved_flag=False)
 
     def resetoutput(self):
         source = self.text.get("iomark", "end-1c")
diff --git a/Lib/idlelib/undo.py b/Lib/idlelib/undo.py
index 4332f1099343b4..ac22e7a291898f 100644
--- a/Lib/idlelib/undo.py
+++ b/Lib/idlelib/undo.py
@@ -47,12 +47,19 @@ def dump_event(self, event):
         pprint(self.undolist[self.pointer:])
         return "break"
 
-    def reset_undo(self):
+    def reset_undo(self, set_saved_flag=True):
+        """Reset the undo/redo history.
+
+        Clear the undo/redo history and optionally set the saved
+        status for the window based on the parameter.  If set_saved_flag
+        is True, then the window will look like it's been saved.  If
+        it's false, it will not be flagged as saved.
+        """
         self.was_saved = -1
         self.pointer = 0
         self.undolist = []
         self.undoblock = 0  # or a CommandSequence instance
-        self.set_saved(1)
+        self.set_saved(set_saved_flag)
 
     def set_saved(self, flag):
         if flag:
@@ -355,6 +362,8 @@ def _undo_delegator(parent):  # htest #
     undo.pack(side='left')
     redo = Button(undowin, text="Redo", command=lambda:d.redo_event(None))
     redo.pack(side='left')
+    reset = Button(undowin, text="Reset Undo", command=lambda:d.reset_undo())
+    reset.pack(side='left')
     dump = Button(undowin, text="Dump", command=lambda:d.dump_event(None))
     dump.pack(side='left')
 
diff --git a/Misc/NEWS.d/next/IDLE/2017-09-22-14-24-52.bpo-21937.jxG9wk.rst b/Misc/NEWS.d/next/IDLE/2017-09-22-14-24-52.bpo-21937.jxG9wk.rst
new file mode 100644
index 00000000000000..82ce90afc272e7
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2017-09-22-14-24-52.bpo-21937.jxG9wk.rst
@@ -0,0 +1 @@
+Fix use of unsaved indicator in titlebar of shell window.