@@ -239,12 +239,16 @@ the :mod:`glob` module.)
 .. function:: isabs(path)
 
    Return ``True`` if *path* is an absolute pathname.  On Unix, that means it
-   begins with a slash, on Windows that it begins with a (back)slash after chopping
-   off a potential drive letter.
+   begins with a slash, on Windows that it begins with two (back)slashes, or a
+   drive letter, colon, and (back)slash together.
 
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
+   .. versionchanged:: 3.13
+      On Windows, returns ``False`` if the given path starts with exactly one
+      (back)slash.
+
 
 .. function:: isfile(path)
 
 
@@ -307,6 +307,13 @@ os
   :c:func:`!posix_spawn_file_actions_addclosefrom_np`.
   (Contributed by Jakub Kulik in :gh:`113117`.)
 
+os.path
+-------
+
+* On Windows, :func:`os.path.isabs` no longer considers paths starting with
+  exactly one (back)slash to be absolute.
+  (Contributed by Barney Gale and Jon Foster in :gh:`44626`.)
+
 pathlib
 -------
 
 
@@ -77,29 +77,22 @@ def normcase(s):
         return s.replace('/', '\\').lower()
 
 
-# Return whether a path is absolute.
-# Trivial in Posix, harder on Windows.
-# For Windows it is absolute if it starts with a slash or backslash (current
-# volume), or if a pathname after the volume-letter-and-colon or UNC-resource
-# starts with a slash or backslash.
-
 def isabs(s):
     """Test whether a path is absolute"""
     s = os.fspath(s)
     if isinstance(s, bytes):
         sep = b'\\'
         altsep = b'/'
         colon_sep = b':\\'
+        double_sep = b'\\\\'
     else:
         sep = '\\'
         altsep = '/'
         colon_sep = ':\\'
+        double_sep = '\\\\'
     s = s[:3].replace(altsep, sep)
     # Absolute: UNC, device, and paths with a drive and root.
-    # LEGACY BUG: isabs("/x") should be false since the path has no drive.
-    if s.startswith(sep) or s.startswith(colon_sep, 1):
-        return True
-    return False
+    return s.startswith(colon_sep, 1) or s.startswith(double_sep)
 
 
 # Join two (or more) paths.
 
@@ -1,5 +1,4 @@
 import functools
-import ntpath
 import posixpath
 from errno import ENOENT, ENOTDIR, EBADF, ELOOP, EINVAL
 from stat import S_ISDIR, S_ISLNK, S_ISREG, S_ISSOCK, S_ISBLK, S_ISCHR, S_ISFIFO
@@ -373,10 +372,7 @@ def parents(self):
     def is_absolute(self):
         """True if the path is absolute (has both a root and, if applicable,
         a drive)."""
-        if self.pathmod is ntpath:
-            # ntpath.isabs() is defective - see GH-44626.
-            return bool(self.drive and self.root)
-        elif self.pathmod is posixpath:
+        if self.pathmod is posixpath:
             # Optimization: work with raw paths on POSIX.
             for path in self._raw_paths:
                 if path.startswith('/'):
 
@@ -227,10 +227,18 @@ def test_split(self):
         tester('ntpath.split("//conky/mountpoint/")', ('//conky/mountpoint/', ''))
 
     def test_isabs(self):
+        tester('ntpath.isabs("foo\\bar")', 0)
+        tester('ntpath.isabs("foo/bar")', 0)
         tester('ntpath.isabs("c:\\")', 1)
+        tester('ntpath.isabs("c:\\foo\\bar")', 1)
+        tester('ntpath.isabs("c:/foo/bar")', 1)
         tester('ntpath.isabs("\\\\conky\\mountpoint\\")', 1)
-        tester('ntpath.isabs("\\foo")', 1)
-        tester('ntpath.isabs("\\foo\\bar")', 1)
+
+        # gh-44626: paths with only a drive or root are not absolute.
+        tester('ntpath.isabs("\\foo\\bar")', 0)
+        tester('ntpath.isabs("/foo/bar")', 0)
+        tester('ntpath.isabs("c:foo\\bar")', 0)
+        tester('ntpath.isabs("c:foo/bar")', 0)
 
         # gh-96290: normal UNC paths and device paths without trailing backslashes
         tester('ntpath.isabs("\\\\conky\\mountpoint")', 1)
 
@@ -1011,10 +1011,14 @@ def test_is_absolute(self):
         self.assertTrue(P('c:/a').is_absolute())
         self.assertTrue(P('c:/a/b/').is_absolute())
         # UNC paths are absolute by definition.
+        self.assertTrue(P('//').is_absolute())
+        self.assertTrue(P('//a').is_absolute())
         self.assertTrue(P('//a/b').is_absolute())
         self.assertTrue(P('//a/b/').is_absolute())
         self.assertTrue(P('//a/b/c').is_absolute())
         self.assertTrue(P('//a/b/c/d').is_absolute())
+        self.assertTrue(P('//?/UNC/').is_absolute())
+        self.assertTrue(P('//?/UNC/spam').is_absolute())
 
     def test_join(self):
         P = self.cls
 
@@ -459,8 +459,8 @@ def _join(name):
 
     def testParseArgsAbsolutePathsThatCannotBeConverted(self):
         program = self.program
-        # even on Windows '/...' is considered absolute by os.path.abspath
-        argv = ['progname', '/foo/bar/baz.py', '/green/red.py']
+        drive = os.path.splitdrive(os.getcwd())[0]
+        argv = ['progname', f'{drive}/foo/bar/baz.py', f'{drive}/green/red.py']
         self._patch_isfile(argv)
 
         program.createTests = lambda: None
 
@@ -36,6 +36,7 @@
 TEMP_DIR = None
 DATA_DIR = pathlib.Path(__file__).parent / "data"
 ZONEINFO_JSON = DATA_DIR / "zoneinfo_data.json"
+DRIVE = os.path.splitdrive('x:')[0]
 
 # Useful constants
 ZERO = timedelta(0)
@@ -1679,8 +1680,8 @@ def test_env_variable(self):
         """Tests that the environment variable works with reset_tzpath."""
         new_paths = [
             ("", []),
-            ("/etc/zoneinfo", ["/etc/zoneinfo"]),
-            (f"/a/b/c{os.pathsep}/d/e/f", ["/a/b/c", "/d/e/f"]),
+            (f"{DRIVE}/etc/zoneinfo", [f"{DRIVE}/etc/zoneinfo"]),
+            (f"{DRIVE}/a/b/c{os.pathsep}{DRIVE}/d/e/f", [f"{DRIVE}/a/b/c", f"{DRIVE}/d/e/f"]),
         ]
 
         for new_path_var, expected_result in new_paths:
@@ -1694,22 +1695,22 @@ def test_env_variable_relative_paths(self):
         test_cases = [
             [("path/to/somewhere",), ()],
             [
-                ("/usr/share/zoneinfo", "path/to/somewhere",),
-                ("/usr/share/zoneinfo",),
+                (f"{DRIVE}/usr/share/zoneinfo", "path/to/somewhere",),
+                (f"{DRIVE}/usr/share/zoneinfo",),
             ],
             [("../relative/path",), ()],
             [
-                ("/usr/share/zoneinfo", "../relative/path",),
-                ("/usr/share/zoneinfo",),
+                (f"{DRIVE}/usr/share/zoneinfo", "../relative/path",),
+                (f"{DRIVE}/usr/share/zoneinfo",),
             ],
             [("path/to/somewhere", "../relative/path",), ()],
             [
                 (
-                    "/usr/share/zoneinfo",
+                    f"{DRIVE}/usr/share/zoneinfo",
                     "path/to/somewhere",
                     "../relative/path",
                 ),
-                ("/usr/share/zoneinfo",),
+                (f"{DRIVE}/usr/share/zoneinfo",),
             ],
         ]
 
@@ -1727,9 +1728,9 @@ def test_env_variable_relative_paths(self):
                     self.assertSequenceEqual(tzpath, expected_paths)
 
     def test_reset_tzpath_kwarg(self):
-        self.module.reset_tzpath(to=["/a/b/c"])
+        self.module.reset_tzpath(to=[f"{DRIVE}/a/b/c"])
 
-        self.assertSequenceEqual(self.module.TZPATH, ("/a/b/c",))
+        self.assertSequenceEqual(self.module.TZPATH, (f"{DRIVE}/a/b/c",))
 
     def test_reset_tzpath_relative_paths(self):
         bad_values = [
@@ -1758,8 +1759,8 @@ def test_tzpath_type_error(self):
                     self.module.reset_tzpath(bad_value)
 
     def test_tzpath_attribute(self):
-        tzpath_0 = ["/one", "/two"]
-        tzpath_1 = ["/three"]
+        tzpath_0 = [f"{DRIVE}/one", f"{DRIVE}/two"]
+        tzpath_1 = [f"{DRIVE}/three"]
 
         with self.tzpath_context(tzpath_0):
             query_0 = self.module.TZPATH
 
@@ -0,0 +1,5 @@
+Fix :func:`os.path.isabs` incorrectly returning ``True`` when given a path
+that starts with exactly one (back)slash on Windows.
+
+Fix :meth:`pathlib.PureWindowsPath.is_absolute` incorrectly returning
+``False`` for some paths beginning with two (back)slashes.