diff --git a/Misc/NEWS.d/next/Library/2024-11-06-12-29-28.gh-issue-126316.X0zQqr.rst b/Misc/NEWS.d/next/Library/2024-11-06-12-29-28.gh-issue-126316.X0zQqr.rst
new file mode 100644
index 00000000000000..52a84138b39339
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-11-06-12-29-28.gh-issue-126316.X0zQqr.rst
@@ -0,0 +1,2 @@
+:mod:`grp`: Fix segfault on concurrent calls on Free Threaded build by using
+critical sections. Patch by Victor Stinner.
diff --git a/Modules/clinic/grpmodule.c.h b/Modules/clinic/grpmodule.c.h
index cc0ad210f42743..ad809ee81a6e1f 100644
--- a/Modules/clinic/grpmodule.c.h
+++ b/Modules/clinic/grpmodule.c.h
@@ -2,6 +2,8 @@
 preserve
 [clinic start generated code]*/
 
+#include "pycore_critical_section.h"// Py_BEGIN_CRITICAL_SECTION()
+
 PyDoc_STRVAR(grp_getgrgid__doc__,
 "getgrgid($module, /, id)\n"
 "--\n"
@@ -26,7 +28,9 @@ grp_getgrgid(PyObject *module, PyObject *args, PyObject *kwargs)
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O:getgrgid", _keywords,
         &id))
         goto exit;
+    Py_BEGIN_CRITICAL_SECTION(module);
     return_value = grp_getgrgid_impl(module, id);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -56,7 +60,9 @@ grp_getgrnam(PyObject *module, PyObject *args, PyObject *kwargs)
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "U:getgrnam", _keywords,
         &name))
         goto exit;
+    Py_BEGIN_CRITICAL_SECTION(module);
     return_value = grp_getgrnam_impl(module, name);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
@@ -80,6 +86,12 @@ grp_getgrall_impl(PyObject *module);
 static PyObject *
 grp_getgrall(PyObject *module, PyObject *Py_UNUSED(ignored))
 {
-    return grp_getgrall_impl(module);
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(module);
+    return_value = grp_getgrall_impl(module);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
 }
-/*[clinic end generated code: output=81f180beb67fc585 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=dfcdb3a6951a1805 input=a9049054013a1b77]*/
diff --git a/Modules/grpmodule.c b/Modules/grpmodule.c
index f7d3e12f347ec2..73bee3ebc7f054 100644
--- a/Modules/grpmodule.c
+++ b/Modules/grpmodule.c
@@ -1,9 +1,17 @@
-/* UNIX group file access module */
+/* UNIX group file access module
+ *
+ * Use @critical_section since getgrgid(), getgrnam() and getgrall() are not
+ * thread safe (gh-126316).
+ */
 
 // Need limited C API version 3.13 for PyMem_RawRealloc()
 #include "pyconfig.h"   // Py_GIL_DISABLED
 #ifndef Py_GIL_DISABLED
-#define Py_LIMITED_API 0x030d0000
+#  define Py_LIMITED_API 0x030d0000
+#else
+#  ifndef Py_BUILD_CORE_BUILTIN
+#    define Py_BUILD_CORE_MODULE 1
+#  endif
 #endif
 
 #include "Python.h"
@@ -111,6 +119,7 @@ mkgrent(PyObject *module, struct group *p)
 }
 
 /*[clinic input]
+@critical_section
 grp.getgrgid
 
     id: object
@@ -122,7 +131,7 @@ If id is not valid, raise KeyError.
 
 static PyObject *
 grp_getgrgid_impl(PyObject *module, PyObject *id)
-/*[clinic end generated code: output=30797c289504a1ba input=15fa0e2ccf5cda25]*/
+/*[clinic end generated code: output=30797c289504a1ba input=f5bf2ce5d1b3d6bc]*/
 {
     PyObject *retval = NULL;
     int nomem = 0;
@@ -191,6 +200,7 @@ grp_getgrgid_impl(PyObject *module, PyObject *id)
 }
 
 /*[clinic input]
+@critical_section
 grp.getgrnam
 
     name: unicode
@@ -202,7 +212,7 @@ If name is not valid, raise KeyError.
 
 static PyObject *
 grp_getgrnam_impl(PyObject *module, PyObject *name)
-/*[clinic end generated code: output=67905086f403c21c input=08ded29affa3c863]*/
+/*[clinic end generated code: output=67905086f403c21c input=da37ffb830c57159]*/
 {
     char *buf = NULL, *buf2 = NULL, *name_chars;
     int nomem = 0;
@@ -269,6 +279,7 @@ grp_getgrnam_impl(PyObject *module, PyObject *name)
 }
 
 /*[clinic input]
+@critical_section
 grp.getgrall
 
 Return a list of all available group entries, in arbitrary order.
@@ -279,7 +290,7 @@ to use YP/NIS and may not be accessible via getgrnam or getgrgid.
 
 static PyObject *
 grp_getgrall_impl(PyObject *module)
-/*[clinic end generated code: output=585dad35e2e763d7 input=d7df76c825c367df]*/
+/*[clinic end generated code: output=585dad35e2e763d7 input=b257403558f07a1d]*/
 {
     PyObject *d;
     struct group *p;