diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
@@ -782,6 +782,23 @@ def test_issue35714(self):
                                         'embedded null character'):
                 struct.calcsize(s)
 
+    def test_non_ascii_bytes_format(self):
+        calls = (
+            (struct.Struct, ()),
+            (struct.calcsize, ()),
+            (struct.pack, ()),
+            (struct.pack_into, (bytearray(), 0)),
+            (struct.unpack, (b'',)),
+            (struct.unpack_from, (b'',)),
+            (struct.iter_unpack, (b'',)),
+        )
+        for format in (b'\xc5', b'<\xc5'):
+            for func, args in calls:
+                with self.subTest(format=format, func=func):
+                    with self.assertRaisesRegex(struct.error,
+                                                'bad char in struct format'):
+                        func(format, *args)
+
     @support.cpython_only
     def test_issue98248(self):
         def test_error_msg(prefix, int_type, is_unsigned):

diff --git a/Misc/NEWS.d/next/Library/2026-09-26-02-04-33.gh-issue-158172.Rn6P4q.rst b/Misc/NEWS.d/next/Library/2026-09-26-02-04-33.gh-issue-158172.Rn6P4q.rst
@@ -0,0 +1,2 @@
+Fix :mod:`struct` functions to raise :exc:`struct.error` instead of
+:exc:`UnicodeEncodeError` for non-ASCII bytes format strings.
diff --git a/Modules/_struct.c b/Modules/_struct.c
@@ -2538,7 +2538,7 @@ cache_struct_converter(PyObject *module, PyObject *fmt, PyStructObject **ptr)
         return Py_CLEANUP_SUPPORTED;
     }
 
-    s_object = PyObject_CallOneArg(state->PyStructType, key);
+    s_object = PyObject_CallOneArg(state->PyStructType, fmt);
     if (s_object != NULL) {
         if (PyDict_GET_SIZE(state->cache) >= MAXCACHE)
             PyDict_Clear(state->cache);
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix :mod:`struct` functions to raise :exc:`struct.error` instead of
		:exc:`UnicodeEncodeError` for non-ASCII bytes format strings.