diff --git a/lld/MachO/Config.h b/lld/MachO/Config.h
--- a/lld/MachO/Config.h
+++ b/lld/MachO/Config.h
@@ -30,6 +30,14 @@
   llvm::VersionTuple sdk;
 };
 
+enum class UndefinedSymbolTreatment : unsigned {
+  unknown = 0,
+  error = 1,
+  warning = 2,
+  suppress = 3,
+  dynamic_lookup = 4,
+};
+
 struct Configuration {
   Symbol *entry;
   bool hasReexports = false;
@@ -52,6 +60,8 @@
   bool demangle = false;
   llvm::MachO::Architecture arch;
   PlatformInfo platform;
+  UndefinedSymbolTreatment undefinedSymbolTreatment =
+      UndefinedSymbolTreatment::error;
   llvm::MachO::HeaderFileType outputType;
   std::vector<llvm::StringRef> systemLibraryRoots;
   std::vector<llvm::StringRef> librarySearchPaths;
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -597,6 +597,19 @@
     error(Twine("malformed sdk version: ") + sdkVersionStr);
 }
 
+static void handleUndefined(const opt::Arg *arg) {
+  StringRef treatmentStr = arg->getValue(0);
+  config->undefinedSymbolTreatment =
+      llvm::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)
+    error(Twine("malformed undefined-symbol treatment: ") + treatmentStr);
+}
+
 static void warnIfDeprecatedOption(const opt::Option &opt) {
   if (!opt.getGroup().isValid())
     return;
@@ -827,6 +840,9 @@
     case OPT_platform_version:
       handlePlatformVersion(arg);
       break;
+    case OPT_undefined:
+      handleUndefined(arg);
+      break;
     default:
       break;
     }
diff --git a/lld/MachO/Options.td b/lld/MachO/Options.td
--- a/lld/MachO/Options.td
+++ b/lld/MachO/Options.td
@@ -435,7 +435,6 @@
 def undefined : Separate<["-"], "undefined">,
      MetaVarName<"<treatment>">,
      HelpText<"Handle undefined symbols according to <treatment>: error, warning, suppress, or dynamic_lookup (default is error)">,
-     Flags<[HelpHidden]>,
      Group<grp_resolve>;
 def rpath : Separate<["-"], "rpath">,
      MetaVarName<"<path>">,
diff --git a/lld/MachO/SymbolTable.h b/lld/MachO/SymbolTable.h
--- a/lld/MachO/SymbolTable.h
+++ b/lld/MachO/SymbolTable.h
@@ -55,6 +55,9 @@
   std::vector<Symbol *> symVector;
 };
 
+extern bool undefinedSymbol(const std::string symbolName,
+                            const std::string referenceName);
+
 extern SymbolTable *symtab;
 
 } // namespace macho
diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "SymbolTable.h"
+#include "Config.h"
 #include "InputFiles.h"
 #include "Symbols.h"
 #include "lld/Common/ErrorHandler.h"
@@ -141,4 +142,27 @@
   return s;
 }
 
+bool lld::macho::undefinedSymbol(const std::string symbolName,
+                                 const std::string referenceName) {
+  std::string message = "undefined symbol " + symbolName;
+  if (referenceName != "")
+    message += ", referenced from " + referenceName;
+  switch (config->undefinedSymbolTreatment) {
+  case UndefinedSymbolTreatment::suppress:
+    return true;
+  case UndefinedSymbolTreatment::error:
+    error(message);
+    return false;
+  case UndefinedSymbolTreatment::warning:
+    warn(message);
+    return true;
+  case UndefinedSymbolTreatment::dynamic_lookup:
+    error("dynamic_lookup unimplemented for " + message);
+    return false;
+  default:
+    fatal("malformed -undefined treatment");
+    return false;
+  }
+}
+
 SymbolTable *macho::symtab;
diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -411,8 +411,7 @@
     for (Reloc &r : isec->relocs) {
       if (auto *s = r.referent.dyn_cast<lld::macho::Symbol *>()) {
         if (isa<Undefined>(s))
-          error("undefined symbol " + toString(*s) + ", referenced from " +
-                toString(isec->file));
+          undefinedSymbol(toString(*s), toString(isec->file));
         else
           target->prepareSymbolRelocation(s, isec, r);
       } else {