diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.h b/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.h
--- a/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.h
@@ -133,6 +133,11 @@
   u32 ConsumeThreadUserId(uptr user_id);
   void SetThreadUserId(u32 tid, uptr user_id);
 
+  // OnFork must be called in the child process after fork to purge old
+  // threads that don't exist anymore (except for the current thread tid).
+  // Returns number of alive threads before fork.
+  u32 OnFork(u32 tid);
+
  private:
   const ThreadContextFactory context_factory_;
   const u32 max_threads_;
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_thread_registry.cpp
@@ -110,7 +110,7 @@
       max_threads_(max_threads),
       thread_quarantine_size_(thread_quarantine_size),
       max_reuse_(max_reuse),
-      mtx_(),
+      mtx_(MutexThreadRegistry),
       total_threads_(0),
       alive_threads_(0),
       max_alive_threads_(0),
@@ -365,4 +365,20 @@
   CHECK(live_.try_emplace(user_id, tctx->tid).second);
 }
 
+u32 ThreadRegistry::OnFork(u32 tid) {
+  ThreadRegistryLock l(this);
+  // We only purge user_id (pthread_t) of live threads because
+  // they cause CHECK failures if new threads with matching pthread_t
+  // created after fork.
+  // Potentially we could purge more info (ThreadContextBase themselves),
+  // but it's hard to test and easy to introduce new issues by doing this.
+  for (auto *tctx : threads_) {
+    if (tctx->tid == tid || !tctx->user_id)
+      continue;
+    CHECK(live_.erase(tctx->user_id));
+    tctx->user_id = 0;
+  }
+  return alive_threads_;
+}
+
 }  // namespace __sanitizer
diff --git a/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp b/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp
--- a/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp
+++ b/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp
@@ -801,7 +801,7 @@
 
 void ForkChildAfter(ThreadState* thr, uptr pc, bool start_thread) {
   ForkAfter(thr);
-  u32 nthread = ThreadCount(thr);
+  u32 nthread = ctx->thread_registry.OnFork(thr->tid);
   VPrintf(1,
           "ThreadSanitizer: forked new process with pid %d,"
           " parent had %d threads\n",
diff --git a/compiler-rt/test/tsan/fork_multithreaded.cpp b/compiler-rt/test/tsan/fork_multithreaded.cpp
--- a/compiler-rt/test/tsan/fork_multithreaded.cpp
+++ b/compiler-rt/test/tsan/fork_multithreaded.cpp
@@ -1,4 +1,4 @@
-// RUN: %clangxx_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s -check-prefix=CHECK-DIE
+// RUN: %clangxx_tsan -O1 %s -o %t && %run %t 66 2>&1 | FileCheck %s -check-prefix=CHECK-DIE
 // RUN: %clangxx_tsan -O1 %s -o %t && %env_tsan_opts=die_after_fork=0 %run %t 2>&1 | FileCheck %s -check-prefix=CHECK-NODIE
 #include "test.h"
 #include <errno.h>
@@ -6,7 +6,7 @@
 #include <sys/wait.h>
 
 static void *sleeper(void *p) {
-  sleep(1000);  // not intended to exit during test
+  barrier_wait(&barrier);
   return 0;
 }
 
@@ -14,31 +14,53 @@
   return 0;
 }
 
-int main() {
-  barrier_init(&barrier, 2);
-  pthread_t th;
-  pthread_create(&th, 0, sleeper, 0);
-  switch (fork()) {
-  default:  // parent
-    while (wait(0) < 0) {}
-    break;
-  case 0:  // child
-    {
-      pthread_t th2;
-      pthread_create(&th2, 0, nop, 0);
-      exit(0);
-      break;
-    }
-  case -1:  // error
+int main(int argc, const char **argv) {
+  barrier_init(&barrier, 3);
+  const int kSleeperThreads = 2;
+  barrier_init(&barrier, kSleeperThreads + 1);
+  pthread_t th0[kSleeperThreads];
+  for (int i = 0; i < kSleeperThreads; i++)
+    pthread_create(&th0[i], 0, sleeper, 0);
+  const int kNopThreads = 5;
+  pthread_t th1[kNopThreads];
+  for (int i = 0; i < kNopThreads; i++)
+    pthread_create(&th1[i], 0, nop, 0);
+  for (int i = 0; i < kNopThreads; i++)
+    pthread_join(th1[i], 0);
+  int pid = fork();
+  if (pid < 0) {
     fprintf(stderr, "failed to fork (%d)\n", errno);
     exit(1);
   }
+  if (pid == 0) {
+    // child
+    const int kChildThreads = 4;
+    pthread_t th2[kChildThreads];
+    for (int i = 0; i < kChildThreads; i++)
+      pthread_create(&th2[i], 0, nop, 0);
+    for (int i = 0; i < kChildThreads; i++)
+      pthread_join(th2[i], 0);
+    exit(0);
+    return 0;
+  }
+  // parent
+  int expect = argc > 1 ? atoi(argv[1]) : 0;
+  int status = 0;
+  while (waitpid(pid, &status, 0) != pid) {
+  }
+  if (!WIFEXITED(status) || WEXITSTATUS(status) != expect) {
+    fprintf(stderr, "subprocess exited with %d, expected %d\n", status, expect);
+    exit(1);
+  }
+  barrier_wait(&barrier);
+  for (int i = 0; i < kSleeperThreads; i++)
+    pthread_join(th0[i], 0);
   fprintf(stderr, "OK\n");
+  return 0;
 }
 
 // CHECK-DIE: ThreadSanitizer: starting new threads after multi-threaded fork is not supported
-// CHECK-DIE: OK
 
-// CHECK-NODIE-NOT: ThreadSanitizer: starting new threads after multi-threaded fork is not supported
+// CHECK-NODIE-NOT: ThreadSanitizer:
 // CHECK-NODIE: OK
-
+// CHECK-NODIE-NOT: ThreadSanitizer: