diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index fb7cfe064e66..48dada7f847f 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,33 @@
+2017-08-10  Yusuke Suzuki  <utatane.tea@gmail.com>
+
+        Make ThreadGlobalData RefCounted for web thread
+        https://bugs.webkit.org/show_bug.cgi?id=175439
+
+        Reviewed by Mark Lam.
+
+        When the web thread is enabled, we share ThreadGlobalData between the web thread and the main thread.
+        The problem happens when the main thread is dying. It could start deallocating TLS and the web
+        thread may see the destructed ThreadGlobalData.
+
+        Even though, the current implementation is safe because the main thread do not perform TLS deallocation
+        in the Darwin environment. But this is not true in Windows. And we should not rely on this condition
+        that depends on the platforms.
+
+        In this patch, we make ThreadGlobalData ThreadSafeRefCounted. This type verbosely describes that
+        ThreadGlobalData could be shared between threads when the web thread enabled. And make the life time
+        management simple instead of relying on the platform dependent TLS implementation.
+
+        * platform/ThreadGlobalData.cpp:
+        (WebCore::ThreadGlobalData::setWebCoreThreadData):
+        (WebCore::threadGlobalData):
+        * platform/ThreadGlobalData.h:
+        (WebCore::ThreadGlobalData::cachedResourceRequestInitiators): Deleted.
+        (WebCore::ThreadGlobalData::eventNames): Deleted.
+        (WebCore::ThreadGlobalData::threadTimers): Deleted.
+        (WebCore::ThreadGlobalData::qualifiedNameCache): Deleted.
+        (WebCore::ThreadGlobalData::cachedConverterICU): Deleted.
+        (WebCore::ThreadGlobalData::cachedConverterTEC): Deleted.
+
 2017-08-10  Yusuke Suzuki  <utatane.tea@gmail.com>
 
         [JSC] Use @toNumber in builtins
diff --git a/Source/WebCore/platform/ThreadGlobalData.cpp b/Source/WebCore/platform/ThreadGlobalData.cpp
index 737867d5334c..fa2da4945b90 100644
--- a/Source/WebCore/platform/ThreadGlobalData.cpp
+++ b/Source/WebCore/platform/ThreadGlobalData.cpp
@@ -81,7 +81,7 @@ void ThreadGlobalData::destroy()
 }
 
 #if USE(WEB_THREAD)
-static ThreadSpecific<std::unique_ptr<ThreadGlobalData>>* staticData { nullptr };
+static ThreadSpecific<RefPtr<ThreadGlobalData>>* staticData { nullptr };
 static ThreadGlobalData* sharedMainThreadStaticData { nullptr };
 
 void ThreadGlobalData::setWebCoreThreadData()
@@ -90,11 +90,7 @@ void ThreadGlobalData::setWebCoreThreadData()
     ASSERT(&threadGlobalData() != sharedMainThreadStaticData);
 
     // Set WebThread's ThreadGlobalData object to be the same as the main UI thread.
-    // The web thread never finishes, and we expect the main thread to also never finish.
-    // Hence, it is safe to store the same ThreadGlobalData pointer in a thread specific std::unique_ptr.
-    // FIXME: Make ThreadGlobalData RefCounted for web thread.
-    // https://bugs.webkit.org/show_bug.cgi?id=175439
-    (**staticData).reset(sharedMainThreadStaticData);
+    **staticData = adoptRef(sharedMainThreadStaticData);
 
     ASSERT(&threadGlobalData() == sharedMainThreadStaticData);
 }
@@ -102,20 +98,22 @@ void ThreadGlobalData::setWebCoreThreadData()
 ThreadGlobalData& threadGlobalData()
 {
     if (UNLIKELY(!staticData)) {
-        staticData = new ThreadSpecific<std::unique_ptr<ThreadGlobalData>>;
+        staticData = new ThreadSpecific<RefPtr<ThreadGlobalData>>;
         auto& result = **staticData;
         ASSERT(!result);
-        result.reset(new ThreadGlobalData());
+        result = adoptRef(new ThreadGlobalData);
         // WebThread and main UI thread need to share the same object. Save it in a static
         // here, the WebThread will pick it up in setWebCoreThreadData().
-        if (pthread_main_np())
+        if (pthread_main_np()) {
             sharedMainThreadStaticData = result.get();
+            result->ref();
+        }
         return *result;
     }
 
     auto& result = **staticData;
     if (!result)
-        result.reset(new ThreadGlobalData());
+        result = adoptRef(new ThreadGlobalData);
     return *result;
 }
 
diff --git a/Source/WebCore/platform/ThreadGlobalData.h b/Source/WebCore/platform/ThreadGlobalData.h
index 7db5b98bc657..6ff0d4abca65 100644
--- a/Source/WebCore/platform/ThreadGlobalData.h
+++ b/Source/WebCore/platform/ThreadGlobalData.h
@@ -27,6 +27,7 @@
 #ifndef ThreadGlobalData_h
 #define ThreadGlobalData_h
 
+#include <wtf/ThreadSafeRefCounted.h>
 #include <wtf/text/StringHash.h>
 
 namespace WebCore {
@@ -39,7 +40,11 @@ namespace WebCore {
     struct ICUConverterWrapper;
     struct TECConverterWrapper;
 
+#if USE(WEB_THREAD)
+    class ThreadGlobalData : public ThreadSafeRefCounted<ThreadGlobalData> {
+#else
     class ThreadGlobalData {
+#endif
         WTF_MAKE_NONCOPYABLE(ThreadGlobalData);
         WTF_MAKE_FAST_ALLOCATED;
     public: