diff --git a/Lib/shlex.py b/Lib/shlex.py
index 5959f52dd12639..1b622166519c87 100644
--- a/Lib/shlex.py
+++ b/Lib/shlex.py
@@ -49,14 +49,14 @@ def __init__(self, instream=None, infile=None, posix=False,
         self.token = ''
         self.filestack = deque()
         self.source = None
+        # _pushback_chars is a push back queue used by lookahead logic
+        self._pushback_chars = deque()
         if not punctuation_chars:
             punctuation_chars = ''
         elif punctuation_chars is True:
             punctuation_chars = '();<>|&'
         self._punctuation_chars = punctuation_chars
         if punctuation_chars:
-            # _pushback_chars is a push back queue used by lookahead logic
-            self._pushback_chars = deque()
             # these chars added because allowed in file names, args, wildcards
             self.wordchars += '~-./*?='
             #remove any punctuation chars from wordchars
@@ -132,7 +132,7 @@ def read_token(self):
         quoted = False
         escapedstate = ' '
         while True:
-            if self.punctuation_chars and self._pushback_chars:
+            if self._pushback_chars:
                 nextchar = self._pushback_chars.pop()
             else:
                 nextchar = self.instream.read(1)
@@ -156,8 +156,18 @@ def read_token(self):
                     else:
                         continue
                 elif nextchar in self.commenters:
-                    self.instream.readline()
-                    self.lineno += 1
+                    if self.posix:
+                        # Consume comment until newline or end of file
+                        while True:
+                            nextchar = self.instream.read(1)
+                            if not nextchar:
+                                break
+                            if nextchar == '\n':
+                                self._pushback_chars.append(nextchar)
+                                break
+                    else:
+                        self.instream.readline()
+                        self.lineno += 1
                 elif self.posix and nextchar in self.escape:
                     escapedstate = 'a'
                     self.state = nextchar
@@ -226,14 +236,23 @@ def read_token(self):
                     else:
                         continue
                 elif nextchar in self.commenters:
-                    self.instream.readline()
-                    self.lineno += 1
                     if self.posix:
+                        # Consume comment until newline or end of file
+                        while True:
+                            nextchar = self.instream.read(1)
+                            if not nextchar:
+                                break
+                            if nextchar == '\n':
+                                self._pushback_chars.append(nextchar)
+                                break
                         self.state = ' '
                         if self.token or (self.posix and quoted):
                             break   # emit current token
                         else:
                             continue
+                    else:
+                        self.instream.readline()
+                        self.lineno += 1
                 elif self.state == 'c':
                     if nextchar in self.punctuation_chars:
                         self.token += nextchar
diff --git a/Lib/test/test_shlex.py b/Lib/test/test_shlex.py
index 2a355abdeeb30f..bdc57c3333d34b 100644
--- a/Lib/test/test_shlex.py
+++ b/Lib/test/test_shlex.py
@@ -368,6 +368,15 @@ def testPunctuationCharsReadOnly(self):
         with self.assertRaises(AttributeError):
             shlex_instance.punctuation_chars = False
 
+    def testNewlineAfterComment(self):
+        """Test that newline after comment is not consumed (POSIX compliance)"""
+        # When whitespace is customized to exclude newlines, newlines should
+        # be treated as tokens, even when following a comment
+        s = shlex.shlex('a # comment \n b', posix=True)
+        s.whitespace = ' '
+        result = list(s)
+        self.assertEqual(result, ['a', '\n', 'b'])
+
     @cpython_only
     def test_lazy_imports(self):
         import_helper.ensure_lazy_imports('shlex', {'collections', 're', 'os'})
diff --git a/Misc/NEWS.d/next/Library/2025-11-15-22-30-13.gh-issue-51338.8XI4Hk.rst b/Misc/NEWS.d/next/Library/2025-11-15-22-30-13.gh-issue-51338.8XI4Hk.rst
new file mode 100644
index 00000000000000..998b26ef5d1fab
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-15-22-30-13.gh-issue-51338.8XI4Hk.rst
@@ -0,0 +1 @@
+Fix :mod:`shlex` to preserve newlines after comments in POSIX mode.