diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
index 9d6ae3e4d00ece..9a3a26a8400844 100644
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -1100,6 +1100,11 @@ def test_large_32b_binunicode8(self):
         self.check_unpickling_error((pickle.UnpicklingError, OverflowError),
                                     dumped)
 
+    def test_large_binstring(self):
+        errmsg = 'BINSTRING pickle has negative byte count'
+        with self.assertRaisesRegex(pickle.UnpicklingError, errmsg):
+            self.loads(b'T\0\0\0\x80')
+
     def test_get(self):
         pickled = b'((lp100000\ng100000\nt.'
         unpickled = self.loads(pickled)
diff --git a/Misc/NEWS.d/next/Library/2025-06-10-00-42-30.gh-issue-135321.UHh9jT.rst b/Misc/NEWS.d/next/Library/2025-06-10-00-42-30.gh-issue-135321.UHh9jT.rst
new file mode 100644
index 00000000000000..9e63d8e28b7696
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-10-00-42-30.gh-issue-135321.UHh9jT.rst
@@ -0,0 +1 @@
+Raise a correct exception for values greater than 0x7fffffff for the ``BINSTRING`` opcode in the C implementation of :mod:`pickle`.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index d260f1a68f8c70..2e74d5688629ff 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -5539,17 +5539,16 @@ static int
 load_counted_binstring(PickleState *st, UnpicklerObject *self, int nbytes)
 {
     PyObject *obj;
-    Py_ssize_t size;
+    long size;
     char *s;
 
     if (_Unpickler_Read(self, st, &s, nbytes) < 0)
         return -1;
 
-    size = calc_binsize(s, nbytes);
+    size = calc_binint(s, nbytes);
     if (size < 0) {
-        PyErr_Format(st->UnpicklingError,
-                     "BINSTRING exceeds system's maximum size of %zd bytes",
-                     PY_SSIZE_T_MAX);
+        PyErr_SetString(st->UnpicklingError,
+                     "BINSTRING pickle has negative byte count");
         return -1;
     }