diff --git a/include/swift/Basic/Lazy.h b/include/swift/Basic/Lazy.h
index 6c899abeeffd6..521227f4db231 100644
--- a/include/swift/Basic/Lazy.h
+++ b/include/swift/Basic/Lazy.h
@@ -24,6 +24,24 @@
 #include "swift/Basic/Malloc.h"
 #include "swift/Basic/type_traits.h"
 
+#if defined(__wasi__)
+// Temporary single-threaded stub. Should be replaced with a thread-safe version
+// as soon as the WASI SDK allows it. See https://bugs.swift.org/browse/SR-12766.
+inline void wasi_call_once(int *flag, void *context, void (*func)(void *)) {
+  switch (*flag) {
+  case 0:
+    *flag = 1;
+    func(context);
+    return;
+  case 1:
+    return;
+  default:
+    assert(false && "wasi_call_once got invalid flag");
+    abort();
+  }
+}
+#endif
+
 namespace swift {
 
 #ifdef __APPLE__
@@ -38,6 +56,10 @@ namespace swift {
   using OnceToken_t = unsigned long;
 # define SWIFT_ONCE_F(TOKEN, FUNC, CONTEXT) \
   _swift_once_f(&TOKEN, CONTEXT, FUNC)
+#elif defined(__wasi__)
+  using OnceToken_t = int;
+# define SWIFT_ONCE_F(TOKEN, FUNC, CONTEXT) \
+  ::wasi_call_once(&TOKEN, CONTEXT, FUNC)
 #else
   using OnceToken_t = std::once_flag;
 # define SWIFT_ONCE_F(TOKEN, FUNC, CONTEXT) \