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
@@ -55,6 +55,26 @@
 #ifdef TSAN_EXTERNAL_HOOKS
 bool OnFinalize(bool failed);
 void OnInitialize();
+#elif SANITIZER_MAC
+#include <dlfcn.h>
+// On Darwin, strong definitions overriding weak ones is only guaranteed within
+// a linkage unit (e.g., when linking .o files).  There is just one case where
+// strong in one linkage unit overrides a weak in another.  That is when the
+// strong dylib/exe was built, it linked against a dylib with a weak-def
+// (__tsan_default_options() relies on this).  We want to use these hooks from
+// other libraries (i.e., the Swift runtime) so we need to use dlsym() to look
+// them up.
+static bool OnFinalize(bool failed) {
+  if (auto ptr = dlsym(RTLD_DEFAULT, "_ZN6__tsan10OnFinalizeEb")) {
+    return reinterpret_cast<bool (*)(bool)>(ptr)(failed);
+  }
+  return failed;
+}
+static void OnInitialize() {
+  if (auto ptr = dlsym(RTLD_DEFAULT, "_ZN6__tsan12OnInitializeEv")) {
+    return reinterpret_cast<void (*)()>(ptr)();
+  }
+}
 #else
 SANITIZER_WEAK_CXX_DEFAULT_IMPL
 bool OnFinalize(bool failed) {
diff --git a/compiler-rt/test/tsan/on_initialize_finalize_hooks.cpp b/compiler-rt/test/tsan/on_initialize_finalize_hooks.cpp
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/tsan/on_initialize_finalize_hooks.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_tsan -O1 %s -o %t.lib -fno-sanitize=thread -shared -DBUILD_LIB=1
+// RUN: %clang_tsan -O1 %s %t.lib -o %t
+// RUN: %run %t | FileCheck %s
+
+// Test that initialization/finalization hooks are called, even when they are
+// not defined in the main executable, but by another another library that
+// doesn't directly link against the TSan runtime.
+
+#include <stdio.h>
+
+#if BUILD_LIB
+namespace __tsan {
+void OnInitialize() {
+  printf("__tsan_on_initialize()\n");
+}
+
+bool OnFinalize(bool failed) {
+  printf("__tsan_on_finalize()\n");
+  return failed;
+}
+}
+#else  // BUILD_LIB
+
+int main() {
+  printf("main()\n");
+  return 0;
+}
+
+#endif  // BUILD_LIB
+
+// CHECK: __tsan_on_initialize()
+// CHECK: main()
+// CHECK: __tsan_on_finalize()