diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 35933e9c8c3a97..59ddf9e0b3c919 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -4273,6 +4273,22 @@ def test_times(self):
             self.assertEqual(times.elapsed, 0)
 
 
+@requires_os_func('fork')
+class ForkTests(unittest.TestCase):
+    def test_fork(self):
+        # bpo-42540: ensure os.fork() with non-default memory allocator does
+        # not crash on exit.
+        code = """if 1:
+            import os
+            from test import support
+            pid = os.fork()
+            if pid != 0:
+                support.wait_process(pid, exitcode=0)
+        """
+        assert_python_ok("-c", code)
+        assert_python_ok("-c", code, PYTHONMALLOC="malloc_debug")
+
+
 # Only test if the C version is provided, otherwise TestPEP519 already tested
 # the pure Python implementation.
 if hasattr(os, "_fspath"):
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-11-15-12-08-27.bpo-42540.V2w107.rst b/Misc/NEWS.d/next/Core and Builtins/2021-11-15-12-08-27.bpo-42540.V2w107.rst
new file mode 100644
index 00000000000000..91160598bd3f41
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-11-15-12-08-27.bpo-42540.V2w107.rst	
@@ -0,0 +1,2 @@
+Fix crash when :func:`os.fork` is called with an active non-default
+memory allocator.
diff --git a/Python/pystate.c b/Python/pystate.c
index 9beefa8e20c444..c3520c336a794e 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -139,11 +139,14 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
     _PyMem_SetDefaultAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
     int interp_mutex = _PyThread_at_fork_reinit(&runtime->interpreters.mutex);
-    int main_interp_id_mutex = _PyThread_at_fork_reinit(&runtime->interpreters.main->id_mutex);
     int xidregistry_mutex = _PyThread_at_fork_reinit(&runtime->xidregistry.mutex);
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
+    /* bpo-42540: id_mutex is freed by _PyInterpreterState_Delete, which does
+     * not force the default allocator. */
+    int main_interp_id_mutex = _PyThread_at_fork_reinit(&runtime->interpreters.main->id_mutex);
+
     if (interp_mutex < 0) {
         Py_FatalError("Can't initialize lock for runtime interpreters");
     }