diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
index bffbcb1a60757d..aa793a2c223de9 100644
--- a/Lib/test/test_struct.py
+++ b/Lib/test/test_struct.py
@@ -552,6 +552,9 @@ def test_count_overflow(self):
         hugecount2 = '{}b{}H'.format(sys.maxsize//2, sys.maxsize//2)
         self.assertRaises(struct.error, struct.calcsize, hugecount2)
 
+        hugecount3 = '{}i{}q'.format(sys.maxsize // 4, sys.maxsize // 8)
+        self.assertRaises(struct.error, struct.calcsize, hugecount3)
+
     def test_trailing_counter(self):
         store = array.array('b', b' '*100)
 
diff --git a/Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst b/Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst
new file mode 100644
index 00000000000000..60a5e4ad1f0ca4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst
@@ -0,0 +1,2 @@
+Avoid undefined behaviour from signed integer overflow when parsing format
+strings in the :mod:`struct` module.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 7d2dfc591a2a58..ae8a8ffb3c005a 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1678,7 +1678,15 @@ prepare_s(PyStructObject *self)
             case 's': _Py_FALLTHROUGH;
             case 'p': len++; ncodes++; break;
             case 'x': break;
-            default: len += num; if (num) ncodes++; break;
+            default:
+                if (num > PY_SSIZE_T_MAX - len) {
+                    goto overflow;
+                }
+                len += num;
+                if (num) {
+                    ncodes++;
+                }
+                break;
         }
 
         itemsize = e->size;