diff --git a/Misc/NEWS.d/next/Library/2020-12-09-15-23-28.bpo-31904.g3k5k3.rst b/Misc/NEWS.d/next/Library/2020-12-09-15-23-28.bpo-31904.g3k5k3.rst
new file mode 100644
index 00000000000..7bca3ed845b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-12-09-15-23-28.bpo-31904.g3k5k3.rst
@@ -0,0 +1 @@
+Define THREAD_STACK_SIZE for VxWorks.
diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index e6910b3083a..ec7d737518b 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -62,6 +62,10 @@
 #   define THREAD_STACK_SIZE    0x800000
 #   endif
 #endif
+#if defined(__VXWORKS__) && defined(THREAD_STACK_SIZE) && THREAD_STACK_SIZE == 0
+#undef  THREAD_STACK_SIZE
+#define THREAD_STACK_SIZE       0x100000
+#endif
 /* for safety, ensure a viable minimum stacksize */
 #define THREAD_STACK_MIN        0x8000  /* 32 KiB */
 #else  /* !_POSIX_THREAD_ATTR_STACKSIZE */