diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-02-21-00-12-24.gh-issue-130115.mF-rP6.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-21-00-12-24.gh-issue-130115.mF-rP6.rst
new file mode 100644
index 00000000000000..124da33f8836f6
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-21-00-12-24.gh-issue-130115.mF-rP6.rst
@@ -0,0 +1 @@
+Fix an issue with thread identifiers being sign-extended on some platforms.
diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index f588b4620da0d3..de95e733ce09f3 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -307,6 +307,24 @@ do_start_joinable_thread(void (*func)(void *), void *arg, pthread_t* out_id)
     return 0;
 }
 
+/* Helper to convert pthread_t to PyThread_ident_t. POSIX allows pthread_t to be
+   non-arithmetic, e.g., musl typedefs it as a pointer. */
+static PyThread_ident_t
+_pthread_t_to_ident(pthread_t value) {
+// Cast through an integer type of the same size to avoid sign-extension.
+#if SIZEOF_PTHREAD_T == SIZEOF_VOID_P
+    return (uintptr_t) value;
+#elif SIZEOF_PTHREAD_T == SIZEOF_LONG
+    return (unsigned long) value;
+#elif SIZEOF_PTHREAD_T == SIZEOF_INT
+    return (unsigned int) value;
+#elif SIZEOF_PTHREAD_T == SIZEOF_LONG_LONG
+    return (unsigned long long) value;
+#else
+#error "Unsupported SIZEOF_PTHREAD_T value"
+#endif
+}
+
 int
 PyThread_start_joinable_thread(void (*func)(void *), void *arg,
                                PyThread_ident_t* ident, PyThread_handle_t* handle) {
@@ -314,9 +332,8 @@ PyThread_start_joinable_thread(void (*func)(void *), void *arg,
     if (do_start_joinable_thread(func, arg, &th)) {
         return -1;
     }
-    *ident = (PyThread_ident_t) th;
+    *ident = _pthread_t_to_ident(th);
     *handle = (PyThread_handle_t) th;
-    assert(th == (pthread_t) *ident);
     assert(th == (pthread_t) *handle);
     return 0;
 }
@@ -329,11 +346,7 @@ PyThread_start_new_thread(void (*func)(void *), void *arg)
         return PYTHREAD_INVALID_THREAD_ID;
     }
     pthread_detach(th);
-#if SIZEOF_PTHREAD_T <= SIZEOF_LONG
-    return (unsigned long) th;
-#else
-    return (unsigned long) *(unsigned long *) &th;
-#endif
+    return (unsigned long) _pthread_t_to_ident(th);;
 }
 
 int
@@ -358,8 +371,7 @@ PyThread_get_thread_ident_ex(void) {
     if (!initialized)
         PyThread_init_thread();
     threadid = pthread_self();
-    assert(threadid == (pthread_t) (PyThread_ident_t) threadid);
-    return (PyThread_ident_t) threadid;
+    return _pthread_t_to_ident(threadid);
 }
 
 unsigned long