diff --git a/.gitignore b/.gitignore
index 7608dc7a..53ab61f7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,7 +16,6 @@ dist/
 downloads/
 eggs/
 .eggs/
-lib/
 lib64/
 parts/
 sdist/
diff --git a/run_release.py b/run_release.py
index 3bcf8b3d..e671dc0e 100755
--- a/run_release.py
+++ b/run_release.py
@@ -375,6 +375,49 @@ def check_cpython_repo_is_clean(db: ReleaseShelf) -> None:
         raise ReleaseException("Git repository is not clean")
 
 
+def check_magic_number(db: ReleaseShelf) -> None:
+    release_tag = db["release"]
+    if release_tag.is_final or release_tag.is_release_candidate:
+
+        def out(msg: str) -> None:
+            raise ReleaseException(msg)
+
+    else:
+
+        def out(msg: str) -> None:
+            print("warning:", msg, file=sys.stderr, flush=True)
+
+    def get_magic(source: Path, regex: re.Pattern[str]) -> str:
+        if m := regex.search(source.read_text()):
+            return m.group("magic")
+
+        out(f"Cannot find magic in {source}, tried {regex.pattern}")
+        return "unknown"
+
+    work_dir = Path(db["git_repo"])
+    magic_actual_file = work_dir / "Include" / "internal" / "pycore_magic_number.h"
+    magic_actual_re = re.compile(
+        r"^#define\s+PYC_MAGIC_NUMBER\s+(?P<magic>\d+)$", re.MULTILINE
+    )
+    magic_actual = get_magic(magic_actual_file, magic_actual_re)
+
+    magic_expected_file = work_dir / "Lib" / "test" / "test_importlib" / "test_util.py"
+    magic_expected_re = re.compile(
+        r"^\s+EXPECTED_MAGIC_NUMBER = (?P<magic>\d+)$", re.MULTILINE
+    )
+    magic_expected = get_magic(magic_expected_file, magic_expected_re)
+
+    if magic_actual == magic_expected:
+        return
+
+    out(
+        f"Magic numbers in {magic_actual_file} ({magic_actual})"
+        f" and {magic_expected_file} ({magic_expected}) don't match."
+    )
+    if not ask_question("Do you want to continue? This will fail tests in RC stage."):
+        raise ReleaseException("Magic numbers don't match!")
+
+
 def prepare_temporary_branch(db: ReleaseShelf) -> None:
     subprocess.check_call(
         ["git", "checkout", "-b", f"branch-{db['release']}"], cwd=db["git_repo"]
@@ -1179,6 +1222,7 @@ def _api_key(api_key: str) -> str:
         ),
         Task(check_buildbots, "Check buildbots are good"),
         Task(check_cpython_repo_is_clean, "Checking Git repository is clean"),
+        Task(check_magic_number, "Checking the magic number is up-to-date"),
         Task(prepare_temporary_branch, "Checking out a temporary release branch"),
         Task(run_blurb_release, "Run blurb release"),
         Task(check_cpython_repo_is_clean, "Checking Git repository is clean"),
diff --git a/tests/magicdata/Include/internal/pycore_magic_number.h b/tests/magicdata/Include/internal/pycore_magic_number.h
new file mode 100644
index 00000000..e7c1893d
--- /dev/null
+++ b/tests/magicdata/Include/internal/pycore_magic_number.h
@@ -0,0 +1,11 @@
+// copied from cpython bd3d31f380cd451a4ab6da5fbfde463fed95b5b5
+// ...
+
+#ifndef Py_INTERNAL_MAGIC_NUMBER_H
+#define Py_INTERNAL_MAGIC_NUMBER_H
+
+#define PYC_MAGIC_NUMBER 3603
+
+#endif  // !Py_INTERNAL_MAGIC_NUMBER_H
+
+// ...
diff --git a/tests/magicdata/Lib/test/test_importlib/test_util.py b/tests/magicdata/Lib/test/test_importlib/test_util.py
new file mode 100644
index 00000000..c6164096
--- /dev/null
+++ b/tests/magicdata/Lib/test/test_importlib/test_util.py
@@ -0,0 +1,7 @@
+# copied from cpython bd3d31f380cd451a4ab6da5fbfde463fed95b5b5
+
+
+class SomeClass:
+    def some_method(self) -> None:
+        EXPECTED_MAGIC_NUMBER = 3495
+        print(EXPECTED_MAGIC_NUMBER)
diff --git a/tests/test_run_release.py b/tests/test_run_release.py
index 7268ccfb..8f545bd0 100644
--- a/tests/test_run_release.py
+++ b/tests/test_run_release.py
@@ -1,6 +1,10 @@
+from pathlib import Path
+from typing import cast
+
 import pytest
 
 import run_release
+from release import ReleaseShelf, Tag
 
 
 @pytest.mark.parametrize(
@@ -22,3 +26,14 @@ def test_invalid_extract_github_owner() -> None:
         "https://example.com",
     ):
         run_release.extract_github_owner("https://example.com")
+
+
+def test_check_magic_number() -> None:
+    db = {
+        "release": Tag("3.13.0rc1"),
+        "git_repo": str(Path(__file__).parent / "magicdata"),
+    }
+    with pytest.raises(
+        run_release.ReleaseException, match="Magic numbers in .* don't match"
+    ):
+        run_release.check_magic_number(cast(ReleaseShelf, db))