diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1472,8 +1472,15 @@
           StringRef symbolName = defined->getName();
           if (config->exportedSymbols.match(symbolName)) {
             if (defined->privateExtern) {
-              warn("cannot export hidden symbol " + symbolName +
-                   "\n>>> defined in " + toString(defined->getFile()));
+              // If it's a weak, it's not "hidden".
+              if (!defined->isWeakDef())
+                warn("cannot export hidden symbol " + symbolName +
+                     "\n>>> defined in " + toString(defined->getFile()));
+              else
+                // When a weakdef private extern symbol is exported,
+                // it becomes "non-private".
+                // This matches LD64's behavior.
+                defined->privateExtern = false;
             }
           } else {
             defined->privateExtern = true;
diff --git a/lld/MachO/MarkLive.cpp b/lld/MachO/MarkLive.cpp
--- a/lld/MachO/MarkLive.cpp
+++ b/lld/MachO/MarkLive.cpp
@@ -67,7 +67,7 @@
         // FIXME: Instead of doing this here, maybe the Driver code doing
         // the matching should add them to explicitUndefineds? Then the
         // explicitUndefineds code below would handle this automatically.
-        assert(!defined->privateExtern &&
+        assert((!defined->privateExtern || defined->isWeakDef()) &&
                "should have been rejected by driver");
         addSym(defined);
         continue;
diff --git a/lld/test/MachO/exported-symbols.s b/lld/test/MachO/exported-symbols.s
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/exported-symbols.s
@@ -0,0 +1,27 @@
+# REQUIRES: x86
+
+# RUN: rm -rf %t && mkdir -p %t
+# RUN: llvm-mc %s -triple=x86_64-apple-macos -filetype=obj -o %t/foo.o
+# RUN: llvm-objdump --macho --syms %t/foo.o | FileCheck %s --check-prefix=PRE-CHECK
+
+# PRE-CHECK: SYMBOL TABLE:
+# PRE-CHECK-NEXT w    F __TEXT,__text __foo
+
+# RUN: %lld -exported_symbol "__foo" %t/foo.o -o %t/a.out
+## Post link check that __foo was exported.
+# RUN: llvm-nm %t/a.out | FileCheck %s  --check-prefix=POST-CHECK
+# POST-CHECK-DAG: T __foo
+# POST-CHECK-DAG: t _main
+.section __TEXT,__text,regular,pure_instructions
+
+.globl  __foo, _main
+.weak_def_can_be_hidden __foo
+__foo:
+  pushq %rbp
+  popq  %rbp
+  retq
+
+
+_main:
+  callq __foo
+  retq