diff --git a/lld/MachO/LTO.cpp b/lld/MachO/LTO.cpp
--- a/lld/MachO/LTO.cpp
+++ b/lld/MachO/LTO.cpp
@@ -79,11 +79,15 @@
     // be removed.
     r.Prevailing = !objSym.isUndefined() && sym->getFile() == &f;
 
-    if (const auto *defined = dyn_cast<Defined>(sym))
+    if (const auto *defined = dyn_cast<Defined>(sym)) {
       r.ExportDynamic =
           defined->isExternal() && !defined->privateExtern && exportDynamic;
-    else if (const auto *common = dyn_cast<CommonSymbol>(sym))
+      r.FinalDefinitionInLinkageUnit =
+          !defined->isExternalWeakDef() && !defined->interposable;
+    } else if (const auto *common = dyn_cast<CommonSymbol>(sym)) {
       r.ExportDynamic = !common->privateExtern && exportDynamic;
+      r.FinalDefinitionInLinkageUnit = true;
+    }
 
     r.VisibleToRegularObj =
         sym->isUsedInRegularObj || (r.Prevailing && r.ExportDynamic);
diff --git a/lld/test/MachO/lto-final-definition.ll b/lld/test/MachO/lto-final-definition.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/lto-final-definition.ll
@@ -0,0 +1,27 @@
+; REQUIRES: x86
+; RUN: rm -rf %t; mkdir %t
+; RUN: llvm-as %s -o %t/test.o
+; RUN: %lld -lSystem -dylib %t/test.o -o %t/test -save-temps
+; RUN: llvm-dis %t/test.0.2.internalize.bc -o - | FileCheck %s
+; RUN: %lld -lSystem -dylib %t/test.o -o %t/flat-namespace.dylib -save-temps \
+; RUN:   -flat_namespace
+; RUN: llvm-dis %t/flat-namespace.dylib.0.2.internalize.bc -o - | FileCheck %s \
+; RUN:   --check-prefix=NO-DSO-LOCAL
+
+;; f() is never dso_local since it is a weak external.
+; CHECK:        define weak_odr void @f()
+; CHECK:        define dso_local void @main()
+
+; NO-DSO-LOCAL: define weak_odr void @f()
+; NO-DSO-LOCAL: define void @main()
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define weak_odr void @f() {
+  ret void
+}
+
+define void @main() {
+  ret void
+}
diff --git a/lld/test/MachO/lto-internalize.ll b/lld/test/MachO/lto-internalize.ll
--- a/lld/test/MachO/lto-internalize.ll
+++ b/lld/test/MachO/lto-internalize.ll
@@ -17,7 +17,7 @@
 
 ;; Check that main is not internalized. This covers the case of bitcode symbols
 ;; referenced by undefined symbols that don't belong to any InputFile.
-; CHECK: define void @main()
+; CHECK: define dso_local void @main()
 
 ;; Check that the foo and bar functions are correctly internalized.
 ; CHECK: define internal void @bar()
@@ -25,7 +25,7 @@
 
 ;; Check that a bitcode symbol that is referenced by a regular object file isn't
 ;; internalized.
-; CHECK: define void @used_in_regular_obj()
+; CHECK: define dso_local void @used_in_regular_obj()
 
 ;; Check that a bitcode symbol that is defined in another bitcode file gets
 ;; internalized.
@@ -51,13 +51,13 @@
 
 ;; Note that only foo() gets internalized here; everything else that isn't
 ;; hidden must be exported.
-; DYN: @comm = common global
+; DYN: @comm = common dso_local global
 ; DYN: @comm_hide = internal global
-; DYN: define void @main()
-; DYN: define void @bar()
+; DYN: define dso_local void @main()
+; DYN: define dso_local void @bar()
 ; DYN: define internal void @foo()
-; DYN: define void @used_in_regular_obj()
-; DYN: define void @baz()
+; DYN: define dso_local void @used_in_regular_obj()
+; DYN: define dso_local void @baz()
 
 ; DYN-SYMS-DAG: (__TEXT,__text) external _bar
 ; DYN-SYMS-DAG: (__TEXT,__text) external _baz