diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 69b4343..b3cb17c 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -788,6 +788,8 @@
     "weak_ptr.h",
   ]
   deps = [
+    ":checks",
+    ":macromagic",
     ":refcount",
     "../api:scoped_refptr",
     "../api:sequence_checker",
diff --git a/rtc_base/weak_ptr.cc b/rtc_base/weak_ptr.cc
index 3bfa71b..cec0808 100644
--- a/rtc_base/weak_ptr.cc
+++ b/rtc_base/weak_ptr.cc
@@ -16,25 +16,19 @@
 namespace rtc {
 namespace internal {
 
-WeakReference::Flag::Flag() : is_valid_(true) {}
-
 void WeakReference::Flag::Invalidate() {
-  RTC_DCHECK(checker_.IsCurrent())
-      << "WeakPtrs must be invalidated on the same sequence.";
+  RTC_DCHECK_RUN_ON(&checker_);
   is_valid_ = false;
 }
 
 bool WeakReference::Flag::IsValid() const {
-  RTC_DCHECK(checker_.IsCurrent())
-      << "WeakPtrs must be checked on the same sequence.";
+  RTC_DCHECK_RUN_ON(&checker_);
   return is_valid_;
 }
 
-WeakReference::Flag::~Flag() {}
-
 WeakReference::WeakReference() {}
 
-WeakReference::WeakReference(const Flag* flag) : flag_(flag) {}
+WeakReference::WeakReference(const RefCountedFlag* flag) : flag_(flag) {}
 
 WeakReference::~WeakReference() {}
 
@@ -55,7 +49,7 @@
 WeakReference WeakReferenceOwner::GetRef() const {
   // If we hold the last reference to the Flag then create a new one.
   if (!HasRefs())
-    flag_ = new RefCountedObject<WeakReference::Flag>();
+    flag_ = new WeakReference::RefCountedFlag();
 
   return WeakReference(flag_.get());
 }
diff --git a/rtc_base/weak_ptr.h b/rtc_base/weak_ptr.h
index 7e75b5b..67b6ec1 100644
--- a/rtc_base/weak_ptr.h
+++ b/rtc_base/weak_ptr.h
@@ -16,9 +16,11 @@
 
 #include "api/scoped_refptr.h"
 #include "api/sequence_checker.h"
+#include "rtc_base/checks.h"
 #include "rtc_base/ref_count.h"
 #include "rtc_base/ref_counted_object.h"
 #include "rtc_base/system/no_unique_address.h"
+#include "rtc_base/thread_annotations.h"
 
 // The implementation is borrowed from chromium except that it does not
 // implement SupportsWeakPtr.
@@ -92,25 +94,28 @@
  public:
   // Although Flag is bound to a specific sequence, it may be
   // deleted from another via base::WeakPtr::~WeakPtr().
-  class Flag : public RefCountInterface {
+  class Flag {
    public:
-    Flag();
+    Flag() = default;
 
     void Invalidate();
     bool IsValid() const;
 
    private:
-    friend class RefCountedObject<Flag>;
+    friend class FinalRefCountedObject<Flag>;
 
-    ~Flag() override;
+    ~Flag() = default;
 
     RTC_NO_UNIQUE_ADDRESS ::webrtc::SequenceChecker checker_{
         webrtc::SequenceChecker::kDetached};
-    bool is_valid_;
+    bool is_valid_ RTC_GUARDED_BY(checker_) = true;
   };
 
+  // `RefCountedFlag` is the reference counted (shared), non-virtual, flag type.
+  using RefCountedFlag = FinalRefCountedObject<Flag>;
+
   WeakReference();
-  explicit WeakReference(const Flag* flag);
+  explicit WeakReference(const RefCountedFlag* flag);
   ~WeakReference();
 
   WeakReference(WeakReference&& other);
@@ -121,7 +126,7 @@
   bool is_valid() const;
 
  private:
-  scoped_refptr<const Flag> flag_;
+  scoped_refptr<const RefCountedFlag> flag_;
 };
 
 class WeakReferenceOwner {
@@ -136,7 +141,7 @@
   void Invalidate();
 
  private:
-  mutable scoped_refptr<RefCountedObject<WeakReference::Flag>> flag_;
+  mutable scoped_refptr<WeakReference::RefCountedFlag> flag_;
 };
 
 // This class simplifies the implementation of WeakPtr's type conversion