diff --git a/lld/MachO/Config.h b/lld/MachO/Config.h
--- a/lld/MachO/Config.h
+++ b/lld/MachO/Config.h
@@ -30,6 +30,11 @@
   llvm::VersionTuple sdk;
 };
 
+enum class NamespaceKind {
+  twolevel,
+  flat,
+};
+
 enum class UndefinedSymbolTreatment {
   unknown,
   error,
@@ -61,6 +66,7 @@
   bool demangle = false;
   llvm::MachO::Architecture arch;
   PlatformInfo platform;
+  NamespaceKind namespaceKind = NamespaceKind::twolevel;
   UndefinedSymbolTreatment undefinedSymbolTreatment =
       UndefinedSymbolTreatment::error;
   llvm::MachO::HeaderFileType outputType;
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -582,18 +582,27 @@
 
 static void handleUndefined(const opt::Arg *arg) {
   StringRef treatmentStr = arg->getValue(0);
-  config->undefinedSymbolTreatment =
+  auto treatment =
       StringSwitch<UndefinedSymbolTreatment>(treatmentStr)
           .Case("error", UndefinedSymbolTreatment::error)
           .Case("warning", UndefinedSymbolTreatment::warning)
           .Case("suppress", UndefinedSymbolTreatment::suppress)
           .Case("dynamic_lookup", UndefinedSymbolTreatment::dynamic_lookup)
           .Default(UndefinedSymbolTreatment::unknown);
-  if (config->undefinedSymbolTreatment == UndefinedSymbolTreatment::unknown) {
+  if (treatment == UndefinedSymbolTreatment::unknown) {
     warn(Twine("unknown -undefined TREATMENT '") + treatmentStr +
          "', defaulting to 'error'");
-    config->undefinedSymbolTreatment = UndefinedSymbolTreatment::error;
+    treatment = UndefinedSymbolTreatment::error;
+  } else if (config->namespaceKind == NamespaceKind::twolevel &&
+             (treatment == UndefinedSymbolTreatment::warning ||
+              treatment == UndefinedSymbolTreatment::suppress)) {
+    if (treatment == UndefinedSymbolTreatment::warning)
+      error("'-undefined warning' only valid with '-flat_namespace'");
+    else
+      error("'-undefined suppress' only valid with '-flat_namespace'");
+    treatment = UndefinedSymbolTreatment::error;
   }
+  config->undefinedSymbolTreatment = treatment;
 }
 
 static void warnIfDeprecatedOption(const opt::Option &opt) {
@@ -760,6 +769,18 @@
   if (const opt::Arg *arg = args.getLastArg(OPT_static, OPT_dynamic))
     config->staticLink = (arg->getOption().getID() == OPT_static);
 
+  if (const opt::Arg *arg =
+          args.getLastArg(OPT_flat_namespace, OPT_twolevel_namespace)) {
+    config->namespaceKind = arg->getOption().getID() == OPT_twolevel_namespace
+                                ? NamespaceKind::twolevel
+                                : NamespaceKind::flat;
+    if (config->namespaceKind == NamespaceKind::flat) {
+      warn("Option '" + arg->getOption().getPrefixedName() +
+           "' is not yet implemented. Stay tuned...");
+      config->namespaceKind = NamespaceKind::twolevel;
+    }
+  }
+
   config->systemLibraryRoots = getSystemLibraryRoots(args);
   config->librarySearchPaths =
       getLibrarySearchPaths(args, config->systemLibraryRoots);
diff --git a/lld/MachO/Options.td b/lld/MachO/Options.td
--- a/lld/MachO/Options.td
+++ b/lld/MachO/Options.td
@@ -448,9 +448,12 @@
      Flags<[HelpHidden]>,
      Group<grp_resolve>;
 def flat_namespace : Flag<["-"], "flat_namespace">,
-     HelpText<"Resolve symbols from all dylibs, both direct & transitive. Do not record source libraries: dyld must re-search at runtime and use the first definition found">,
+     HelpText<"Resolve symbols from all dylibs, both direct and transitive. Do not record source libraries: dyld must re-search at runtime and use the first definition found">,
      Flags<[HelpHidden]>,
      Group<grp_resolve>;
+def twolevel_namespace : Flag<["-"], "twolevel_namespace">,
+     HelpText<"Make dyld look up symbols by (dylib,name) pairs (default)">,
+     Group<grp_resolve>;
 def u : Separate<["-"], "u">,
      MetaVarName<"<symbol>">,
      HelpText<"Require that <symbol> be defined for the link to succeed">,
@@ -1302,10 +1305,6 @@
      HelpText<"This option is undocumented in ld64">,
      Flags<[HelpHidden]>,
      Group<grp_undocumented>;
-def twolevel_namespace : Flag<["-"], "twolevel_namespace">,
-     HelpText<"This option is undocumented in ld64">,
-     Flags<[HelpHidden]>,
-     Group<grp_undocumented>;
 def verbose_optimization_hints : Flag<["-"], "verbose_optimization_hints">,
      HelpText<"This option is undocumented in ld64">,
      Flags<[HelpHidden]>,
diff --git a/lld/test/MachO/treat-undef-sym.s b/lld/test/MachO/treat-undef-sym.s
--- a/lld/test/MachO/treat-undef-sym.s
+++ b/lld/test/MachO/treat-undef-sym.s
@@ -5,14 +5,29 @@
 # RUN:     FileCheck %s -check-prefix=UNKNOWN
 # RUN: not %lld -undefined error -o /dev/null %t.o 2>&1 | \
 # RUN:     FileCheck %s -check-prefix=ERROR
-# RUN:     %no_fatal_warnings_lld -undefined warning -o /dev/null %t.o 2>&1 | \
-# RUN:     FileCheck %s -check-prefix=WARNING
-# RUN:     %lld -undefined suppress -o /dev/null %t.o 2>&1 | \
-# RUN:     FileCheck %s -check-prefix=SUPPRESS --allow-empty
+
+# RUN: not %lld -undefined warning -o /dev/null %t.o 2>&1 | \
+# RUN:     FileCheck %s -check-prefix=INVAL-WARNING
+# RUN: not %lld -undefined suppress -o /dev/null %t.o 2>&1 | \
+# RUN:     FileCheck %s -check-prefix=INVAL-SUPPRESS
+
+# FIXME: Enable these -undefined checks once -flat_namespace is implemented.
+# RN: %no_fatal_warnings_lld -flat_namespace -undefined warning \
+# RN:     -o /dev/null %t.o 2>&1 | \
+# RN:     FileCheck %s -check-prefix=WARNING
+# RN: %lld -flat_namespace -undefined suppress -o /dev/null %t.o 2>&1 | \
+# RN:     FileCheck %s -check-prefix=SUPPRESS --allow-empty
 
 # ERROR: error: undefined symbol: _bar
 # ERROR-NEXT: >>> referenced by
 
+# INVAL-WARNING: error: '-undefined warning' only valid with '-flat_namespace'
+# INVAL-WARNING-NEXT: error: undefined symbol: _bar
+
+# INVAL-SUPPRESS: error: '-undefined suppress' only valid with '-flat_namespace'
+# INVAL-SUPPRESS-NEXT: error: undefined symbol: _bar
+
+
 # WARNING: warning: undefined symbol: _bar
 # WARNING-NEXT: >>> referenced by