diff --git a/library/random.system_rng.m b/library/random.system_rng.m
index 96dea968e..a8d084372 100644
--- a/library/random.system_rng.m
+++ b/library/random.system_rng.m
@@ -119,6 +119,9 @@
 
 #include <errno.h>
 #include <stdint.h>
+
+// On Windows we need to ensure that _CRT_RAND_S is defined before stdlib.h
+// is included but this must be done in runtime/mercury_std.h.
 #include <stdlib.h>
 
 // The following macros define if the system random number exists on this
@@ -136,7 +139,7 @@
 //    PRNG, such as ChaCha20; it should _not_ be enabled on systems where
 //    arc4random() still uses RC4.
 //
-// ML_SYSRAND_IMPL_RAND_S (NYI)
+// ML_SYSRAND_IMPL_RAND_S
 //    the system RNG is implemented by calling the rand_s() function
 //    (Windows only).
 //
@@ -169,6 +172,8 @@
    #else
       #define ML_SYSRAND_IMPL_URANDOM
    #endif
+#elif defined(MR_WIN32)
+    #define ML_SYSRAND_IMPL_RAND_S
 #else
     #define ML_SYSRAND_IMPL_NONE
 #endif
@@ -675,6 +680,58 @@ ML_random_generate_bytes(ML_SystemRandomHandle handle,
     return MR_TRUE;
 }
 
+#elif defined(ML_SYSRAND_IMPL_RAND_S)
+
+ML_SystemRandomHandle
+ML_random_open(MR_Bool *succeeded, MR_String *err_msg)
+{
+    *succeeded = MR_TRUE;
+    *err_msg = MR_make_string_const(\"\");
+    return 0;
+}
+
+MR_Bool
+ML_random_close(ML_SystemRandomHandle handle, MR_String *err_msg)
+{
+    *err_msg = MR_make_string_const(\"\");
+    return MR_TRUE;
+}
+
+MR_Bool
+ML_random_generate_bytes(ML_SystemRandomHandle handle,
+    unsigned char *buffer, size_t len, MR_String *err_msg)
+{
+    int err;
+    unsigned int n;
+    size_t num_to_read = len;
+
+    while (num_to_read > 0) {
+        if (num_to_read < 4) {
+            err = rand_s(&n);
+            if (err != 0) {
+                goto rand_s_failure_handler;
+            }
+            MR_memcpy(buffer, (unsigned char *) &n, num_to_read);
+            break;
+        } else {
+            err = rand_s((unsigned int *) buffer);
+            if (err != 0) {
+                goto rand_s_failure_handler;
+            }
+            num_to_read -= 4;
+            buffer += 4;
+        }
+    }
+
+    *err_msg = MR_make_string_const(\"\");
+    return MR_TRUE;
+
+rand_s_failure_handler:
+
+    *err_msg = MR_make_string_const(\"rand_s failed\");
+    return MR_FALSE;
+}
+
 #else // ML_SYSRAND_IMPL_NONE
 
 ML_SystemRandomHandle
diff --git a/runtime/mercury_std.h b/runtime/mercury_std.h
index e34d0df36..5cdb77434 100644
--- a/runtime/mercury_std.h
+++ b/runtime/mercury_std.h
@@ -1,7 +1,7 @@
 // vim: ts=4 sw=4 expandtab ft=c
 
 // Copyright (C) 1993-1995, 1997-2005, 2011-2012 The University of Melbourne.
-// Copyright (C) 2014, 2016-2018 The Mercury team.
+// Copyright (C) 2014, 2016-2019, 2021 The Mercury team.
 // This file is distributed under the terms specified in COPYING.LIB.
 
 // std.h - "standard" [sic] definitions for C:
@@ -22,7 +22,14 @@
     #error "Mercury requires a system that provides stdint.h"
 #endif
 
+// On Windows we need to define _CRT_RAND_S *before* stdlib.h is included,
+// otherwise the declaration for rand_s() will not be visible.
+//
+#if defined(MR_WIN32) && !defined(_CRT_RAND_S)
+   #define _CRT_RAND_S
+#endif
 #include <stdlib.h> // for size_t
+
 #include <assert.h> // for assert()
 #include <errno.h>  // for EINTR
 #include <ctype.h>  // for isalnum(), etc.