diff --git a/stdlib/public/runtime/ThreadLocalStorage.h b/stdlib/public/runtime/ThreadLocalStorage.h
index c4b63fac0aa36..1db5f70b64818 100644
--- a/stdlib/public/runtime/ThreadLocalStorage.h
+++ b/stdlib/public/runtime/ThreadLocalStorage.h
@@ -90,7 +90,10 @@ static_assert(std::is_same<__swift_thread_key_t, DWORD>::value,
 #  define SWIFT_THREAD_KEY_CREATE _stdlib_thread_key_create
 #  define SWIFT_THREAD_GETSPECIFIC FlsGetValue
 #  define SWIFT_THREAD_SETSPECIFIC(key, value) (FlsSetValue(key, value) == FALSE)
-
+# elif defined(__wasi__)
+#  define SWIFT_THREAD_KEY_CREATE _stdlib_thread_key_create
+#  define SWIFT_THREAD_GETSPECIFIC _stdlib_thread_getspecific
+#  define SWIFT_THREAD_SETSPECIFIC _stdlib_thread_setspecific
 # else
 // Otherwise use the pthread API.
 #  include <pthread.h>
diff --git a/stdlib/public/stubs/ThreadLocalStorage.cpp b/stdlib/public/stubs/ThreadLocalStorage.cpp
index 50168f955bc6c..2f4d74d61d26a 100644
--- a/stdlib/public/stubs/ThreadLocalStorage.cpp
+++ b/stdlib/public/stubs/ThreadLocalStorage.cpp
@@ -54,6 +54,44 @@ _stdlib_thread_key_create(__swift_thread_key_t * _Nonnull key,
 
 #endif
 
+#if defined(__wasi__)
+// WebAssembly doesn't have threading standardized yet, so TLS is emulated
+// here, see https://bugs.swift.org/browse/SR-12774.
+#include <map>
+using __swift_thread_key_destructor = void (*)(void *);
+
+struct _stdlib_tls_element_t {
+  const void *value;
+  __swift_thread_key_destructor destructor;
+};
+
+using _stdlib_tls_map_t = std::map<__swift_thread_key_t, _stdlib_tls_element_t>;
+static void *_stdlib_tls_map;
+
+static inline int _stdlib_thread_key_create(__swift_thread_key_t *key,
+                          __swift_thread_key_destructor destructor) {
+  if (!_stdlib_tls_map)
+      _stdlib_tls_map = new _stdlib_tls_map_t();
+  auto *map = (_stdlib_tls_map_t *)_stdlib_tls_map;
+  *key = map->size();
+  _stdlib_tls_element_t element = { nullptr, destructor };
+  map->insert(std::make_pair(*key, element));
+  return 0;
+}
+
+static inline void *_stdlib_thread_getspecific(__swift_thread_key_t key) {
+  auto *map = (_stdlib_tls_map_t *)_stdlib_tls_map;
+  return const_cast<void *>(map->operator[](key).value);
+}
+
+static inline int _stdlib_thread_setspecific(__swift_thread_key_t key, const void *value) {
+  auto *map = (_stdlib_tls_map_t *)_stdlib_tls_map;
+  map->operator[](key).value = value;
+  return 0;
+}
+
+#endif
+
 #if SWIFT_TLS_HAS_RESERVED_PTHREAD_SPECIFIC
 
 SWIFT_RUNTIME_STDLIB_INTERNAL