diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index e7cbfe17ecb3c8..21a56cf47d9f79 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -842,24 +842,24 @@ def relpath(path, start=None):
         raise
 
 
-# Return the longest common sub-path of the sequence of paths given as input.
+# Return the longest common sub-path of the iterable of paths given as input.
 # The function is case-insensitive and 'separator-insensitive', i.e. if the
 # only difference between two paths is the use of '\' versus '/' as separator,
 # they are deemed to be equal.
 #
 # However, the returned path will have the standard '\' separator (even if the
 # given paths had the alternative '/' separator) and will have the case of the
-# first path given in the sequence. Additionally, any trailing separator is
+# first path given in the iterable. Additionally, any trailing separator is
 # stripped from the returned path.
 
 def commonpath(paths):
-    """Given a sequence of path names, returns the longest common sub-path."""
-
+    """Given an iterable of path names, returns the longest common sub-path."""
+    paths = tuple(map(os.fspath, paths))
     if not paths:
-        raise ValueError('commonpath() arg is an empty sequence')
+        raise ValueError('commonpath() arg is an empty iterable')
 
-    paths = tuple(map(os.fspath, paths))
-    if isinstance(paths[0], bytes):
+    path = paths[0]
+    if isinstance(path, bytes):
         sep = b'\\'
         altsep = b'/'
         curdir = b'.'
@@ -869,37 +869,34 @@ def commonpath(paths):
         curdir = '.'
 
     try:
-        drivesplits = [splitroot(p.replace(altsep, sep).lower()) for p in paths]
-        split_paths = [p.split(sep) for d, r, p in drivesplits]
-
-        if len({r for d, r, p in drivesplits}) != 1:
-            raise ValueError("Can't mix absolute and relative paths")
-
-        # Check that all drive letters or UNC paths match. The check is made only
-        # now otherwise type errors for mixing strings and bytes would not be
-        # caught.
-        if len({d for d, r, p in drivesplits}) != 1:
-            raise ValueError("Paths don't have the same drive")
-
-        drive, root, path = splitroot(paths[0].replace(altsep, sep))
-        common = path.split(sep)
-        common = [c for c in common if c and c != curdir]
-
-        split_paths = [[c for c in s if c and c != curdir] for s in split_paths]
-        s1 = min(split_paths)
-        s2 = max(split_paths)
-        for i, c in enumerate(s1):
-            if c != s2[i]:
-                common = common[:i]
-                break
-        else:
-            common = common[:len(s1)]
-
-        return drive + root + sep.join(common)
+        rootsplits = [splitroot(p.replace(altsep, sep).lower()) for p in paths]
     except (TypeError, AttributeError):
         genericpath._check_arg_types('commonpath', *paths)
         raise
 
+    # Check that all drive letters or UNC paths match. The check is made only
+    # now otherwise type errors for mixing strings and bytes would not be
+    # caught.
+    if len({drt[0] for drt in rootsplits}) != 1:
+        raise ValueError("Paths don't have the same drive")
+
+    if len({drt[1] for drt in rootsplits}) != 1:
+        raise ValueError("Can't mix absolute and relative paths")
+
+    drive, root, tail = splitroot(path.replace(altsep, sep))
+    common = [c for c in tail.split(sep) if c and c != curdir]
+    split_paths = [
+        [c for c in drt[2].split(sep) if c and c != curdir]
+        for drt in rootsplits
+    ]
+    s1 = min(split_paths)
+    s2 = max(split_paths)
+    for i, c in enumerate(s1):
+        if c != s2[i]:
+            return drive + root + sep.join(common[:i])
+
+    return drive + root + sep.join(common[:len(s1)])
+
 
 try:
     # The isdir(), isfile(), islink() and exists() implementations in
diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index 33943b4403636a..f1eb7b1d895872 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -432,7 +432,7 @@ def realpath(filename, *, strict=False):
     """Return the canonical path of the specified filename, eliminating any
 symbolic links encountered in the path."""
     filename = os.fspath(filename)
-    path, ok = _joinrealpath(filename[:0], filename, strict, {})
+    path, _ = _joinrealpath(filename[:0], filename, strict, {})
     return abspath(path)
 
 # Join two paths, normalizing and eliminating any symbolic links
@@ -538,45 +538,45 @@ def relpath(path, start=None):
         raise
 
 
-# Return the longest common sub-path of the sequence of paths given as input.
+# Return the longest common sub-path of the iterable of paths given as input.
 # The paths are not normalized before comparing them (this is the
 # responsibility of the caller). Any trailing separator is stripped from the
 # returned path.
 
 def commonpath(paths):
-    """Given a sequence of path names, returns the longest common sub-path."""
+    """Given an iterable of path names, returns the longest common sub-path."""
+    try:
+        # Raises TypeError if paths is not iterable
+        _, roots, tails = zip(*map(splitroot, paths))
+    except ValueError:
+        raise ValueError('commonpath() arg is an empty iterable') from None
 
-    paths = tuple(map(os.fspath, paths))
+    try:
+        root = min(roots)
+    except (TypeError, AttributeError):
+        genericpath._check_arg_types('commonpath', *(
+            # Can't use paths, can be an iterable
+            root + tail for root, tail in zip(roots, tails)
+        ))
+        raise
 
-    if not paths:
-        raise ValueError('commonpath() arg is an empty sequence')
+    if not root and max(roots):
+        raise ValueError("Can't mix absolute and relative paths")
 
-    if isinstance(paths[0], bytes):
+    if isinstance(root, bytes):
         sep = b'/'
         curdir = b'.'
     else:
         sep = '/'
         curdir = '.'
 
-    try:
-        split_paths = [path.split(sep) for path in paths]
+    split_paths = [
+        [c for c in tail.split(sep) if c and c != curdir] for tail in tails
+    ]
+    s1 = min(split_paths)
+    s2 = max(split_paths)
+    for i, c in enumerate(s1):
+        if c != s2[i]:
+            return root + sep.join(s1[:i])
 
-        try:
-            isabs, = set(p[:1] == sep for p in paths)
-        except ValueError:
-            raise ValueError("Can't mix absolute and relative paths") from None
-
-        split_paths = [[c for c in s if c and c != curdir] for s in split_paths]
-        s1 = min(split_paths)
-        s2 = max(split_paths)
-        common = s1
-        for i, c in enumerate(s1):
-            if c != s2[i]:
-                common = s1[:i]
-                break
-
-        prefix = sep if isabs else sep[:0]
-        return prefix + sep.join(common)
-    except (TypeError, AttributeError):
-        genericpath._check_arg_types('commonpath', *paths)
-        raise
+    return root + sep.join(s1)
diff --git a/Lib/test/test_ntpath.py b/Lib/test/test_ntpath.py
index 9cb03e3cd5de8d..c816f99e7e9f1b 100644
--- a/Lib/test/test_ntpath.py
+++ b/Lib/test/test_ntpath.py
@@ -871,11 +871,14 @@ def check_error(exc, paths):
             self.assertRaises(exc, ntpath.commonpath,
                               [os.fsencode(p) for p in paths])
 
+        self.assertRaises(TypeError, ntpath.commonpath, None)
         self.assertRaises(ValueError, ntpath.commonpath, [])
+        self.assertRaises(ValueError, ntpath.commonpath, iter([]))
         check_error(ValueError, ['C:\\Program Files', 'Program Files'])
         check_error(ValueError, ['C:\\Program Files', 'C:Program Files'])
         check_error(ValueError, ['\\Program Files', 'Program Files'])
         check_error(ValueError, ['Program Files', 'C:\\Program Files'])
+
         check(['C:\\Program Files'], 'C:\\Program Files')
         check(['C:\\Program Files', 'C:\\Program Files'], 'C:\\Program Files')
         check(['C:\\Program Files\\', 'C:\\Program Files'],
diff --git a/Lib/test/test_posixpath.py b/Lib/test/test_posixpath.py
index cbb7c4c52d9697..996157a7d867e4 100644
--- a/Lib/test/test_posixpath.py
+++ b/Lib/test/test_posixpath.py
@@ -749,6 +749,9 @@ def check_error(exc, paths):
         self.assertRaises(TypeError, posixpath.commonpath,
                           ['usr/lib/', b'/usr/lib/python3'])
 
+        # gh-117201: Handle leading `//` for `posixpath.commonpath`
+        check(['//foo/bar', '//foo/baz'], '//foo')
+
 
 class PosixCommonTest(test_genericpath.CommonTest, unittest.TestCase):
     pathmodule = posixpath
diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-03-26-20-20-36.gh-issue-117201.qaS7em.rst b/Misc/NEWS.d/next/Core and Builtins/2024-03-26-20-20-36.gh-issue-117201.qaS7em.rst
new file mode 100644
index 00000000000000..cb232bec3a593d
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2024-03-26-20-20-36.gh-issue-117201.qaS7em.rst	
@@ -0,0 +1,2 @@
+* Handle leading ``//`` for :func:`posixpath.commonpath` using :func:`posixpath.splitroot`.
+* Raise TypeError for non-sequences for :func:`ntpath.commonpath`.