diff --git a/src/click/_termui_impl.py b/src/click/_termui_impl.py
index 70aabf8f1..63831ede1 100644
--- a/src/click/_termui_impl.py
+++ b/src/click/_termui_impl.py
@@ -701,7 +701,17 @@ def get_editor(self) -> str:
             if rv:
                 return rv
         if WIN:
-            return "notepad"
+            from shutil import which
+
+            notepad = which("notepad.exe") or which("notepad")
+            if notepad is not None:
+                return notepad
+            windir = (
+                os.environ.get("SystemRoot")
+                or os.environ.get("WINDIR")
+                or "C:\\Windows"
+            )
+            return str(Path(windir) / "System32" / "notepad.exe")
 
         from shutil import which
 
@@ -712,7 +722,6 @@ def get_editor(self) -> str:
 
     def edit_files(self, filenames: cabc.Iterable[str | os.PathLike[str]]) -> None:
         """Open files in the user's editor."""
-        import shlex
         import subprocess
 
         editor = self.get_editor()
@@ -725,9 +734,26 @@ def edit_files(self, filenames: cabc.Iterable[str | os.PathLike[str]]) -> None:
         try:
             # Split in POSIX mode (the default) for the same reasons as
             # in pager(): strips quotes from tokens and preserves quoted
-            # Windows paths.
+            # Windows paths. On Windows, unquoted paths such as the
+            # default notepad.exe under System32 must use non-POSIX
+            # splitting — POSIX mode treats backslashes as escapes and
+            # CreateProcess can fail with WinError 87/14001.
+            editor_lower = editor.lower()
+            file_args = [os.fspath(name) for name in filenames]
+            if WIN and (
+                editor_lower in {"notepad", "notepad.exe"}
+                or editor_lower.endswith(("\\notepad.exe", "/notepad.exe"))
+                or (
+                    len(editor) >= 3
+                    and editor[1:3] == ":\\"
+                    and editor[:1] not in "'\""
+                )
+            ):
+                args = shlex.split(editor, posix=False) + file_args
+            else:
+                args = shlex.split(editor) + file_args
             c = subprocess.Popen(
-                args=shlex.split(editor) + list(filenames),
+                args=args,
                 env=environ,
             )
             exit_code = c.wait()
diff --git a/tests/test_termui.py b/tests/test_termui.py
index 1825b1987..5d0c20add 100644
--- a/tests/test_termui.py
+++ b/tests/test_termui.py
@@ -594,6 +594,55 @@ def test_editor_windows_path_normalization(editor_cmd, expected_cmd):
         assert mock_popen.call_args[1].get("shell") is None
 
 
+def test_get_editor_windows_resolves_notepad(monkeypatch):
+    """Issue #3840: default Windows editor must be a real notepad.exe path."""
+    monkeypatch.setattr(click._termui_impl, "WIN", True)
+    monkeypatch.delenv("VISUAL", raising=False)
+    monkeypatch.delenv("EDITOR", raising=False)
+    monkeypatch.setenv("SystemRoot", "C:\\Windows")
+
+    with patch("shutil.which", return_value=None):
+        editor = Editor().get_editor()
+
+    expected = str(pathlib.Path("C:\\Windows") / "System32" / "notepad.exe")
+    assert editor.lower().endswith("notepad.exe")
+    assert editor == expected
+
+
+def test_edit_files_windows_notepad_full_path_argv(monkeypatch):
+    """Issue #3840: unquoted System32 notepad path must not be POSIX-mangled."""
+    monkeypatch.setattr(click._termui_impl, "WIN", True)
+    notepad = "C:\\Windows\\System32\\notepad.exe"
+
+    with patch("subprocess.Popen") as mock_popen:
+        mock_popen.return_value.wait.return_value = 0
+        Editor(editor=notepad).edit_files(["f.txt"])
+
+        args = mock_popen.call_args[1].get("args") or mock_popen.call_args[0][0]
+        assert args == [notepad, "f.txt"]
+        assert mock_popen.call_args[1].get("shell") is None
+
+
+def test_edit_files_windows_default_notepad_argv(monkeypatch):
+    """Issue #3840: default editor launch uses argv[0] ending in notepad.exe."""
+    monkeypatch.setattr(click._termui_impl, "WIN", True)
+    monkeypatch.delenv("VISUAL", raising=False)
+    monkeypatch.delenv("EDITOR", raising=False)
+    monkeypatch.setenv("SystemRoot", "C:\\Windows")
+
+    with patch("shutil.which", return_value=None):
+        with patch("subprocess.Popen") as mock_popen:
+            mock_popen.return_value.wait.return_value = 0
+            Editor().edit_files(["f.txt"])
+
+            args = mock_popen.call_args[1].get("args") or mock_popen.call_args[0][0]
+            expected = str(pathlib.Path("C:\\Windows") / "System32" / "notepad.exe")
+            assert args[0].lower().endswith("notepad.exe")
+            assert args[0] == expected
+            assert args[1:] == ["f.txt"]
+            assert mock_popen.call_args[1].get("shell") is None
+
+
 def test_editor_env_passed_through():
     with patch("subprocess.Popen") as mock_popen:
         mock_popen.return_value.wait.return_value = 0