diff --git a/llvm/include/llvm/ADT/Triple.h b/llvm/include/llvm/ADT/Triple.h
--- a/llvm/include/llvm/ADT/Triple.h
+++ b/llvm/include/llvm/ADT/Triple.h
@@ -503,6 +503,10 @@
     return getOS() == Triple::FreeBSD;
   }
 
+  bool isOSBSDLike() const {
+    return isOSNetBSD() || isOSOpenBSD() || isOSFreeBSD();
+  }
+
   bool isOSFuchsia() const {
     return getOS() == Triple::Fuchsia;
   }
diff --git a/llvm/test/tools/llvm-ar/lto-kind-from-triple.test b/llvm/test/tools/llvm-ar/lto-kind-from-triple.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-ar/lto-kind-from-triple.test
@@ -0,0 +1,30 @@
+Ensure that we generate a GNU style archive if the first input is a bitcode
+file with a GNU target triple (absence of __.SYMDEF in the archive).
+
+RUN: echo -e "target triple = \"x86_64-unknown-linux-gnu\" \n define void @_Z3foov() { ret void }" > %t.gnu.ll
+RUN: llvm-as -o %t.o %t.gnu.ll
+
+RUN: rm -f %t.ar
+RUN: llvm-ar crs %t.ar %t.o
+RUN: not grep -q __.SYMDEF %t.ar
+
+Ensure that we generate a MachO style archive if the first input is a bitcode
+file with a MachO target triple (presence of __.SYMDEF in the archive).
+
+RUN: echo -e "target triple = \"x86_64-apple-macosx10.9\" \n define void @_Z3foov() { ret void }" > %t.macho.ll
+RUN: llvm-as -o %t.o %t.macho.ll
+
+RUN: rm -f %t.ar
+RUN: llvm-ar crs %t.ar %t.o
+RUN: grep -q __.SYMDEF %t.ar
+
+Ensure that we generate a BSD style archive if the first input is a bitcode
+file with a BSD target triple (presence of __.SYMDEF in the archive).
+
+RUN: echo -e "target triple = \"x86_64-unknown-freebsd\" \n define void @_Z3foov() { ret void }" > %t.bsd.ll
+RUN: llvm-as -o %t.o %t.bsd.ll
+
+RUN: rm -f %t.ar
+RUN: llvm-ar crs %t.ar %t.o
+RUN: grep -q __.SYMDEF %t.ar
+
diff --git a/llvm/tools/llvm-ar/llvm-ar.cpp b/llvm/tools/llvm-ar/llvm-ar.cpp
--- a/llvm/tools/llvm-ar/llvm-ar.cpp
+++ b/llvm/tools/llvm-ar/llvm-ar.cpp
@@ -14,11 +14,14 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Triple.h"
+#include "llvm/BinaryFormat/Magic.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/ArchiveWriter.h"
+#include "llvm/Object/IRObjectFile.h"
 #include "llvm/Object/MachO.h"
 #include "llvm/Object/ObjectFile.h"
+#include "llvm/Object/SymbolicFile.h"
 #include "llvm/Support/Chrono.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ConvertUTF.h"
@@ -875,8 +878,9 @@
 }
 
 static object::Archive::Kind getKindFromMember(const NewArchiveMember &Member) {
+  auto MemBufferRef = Member.Buf->getMemBufferRef();
   Expected<std::unique_ptr<object::ObjectFile>> OptionalObject =
-      object::ObjectFile::createObjectFile(Member.Buf->getMemBufferRef());
+      object::ObjectFile::createObjectFile(MemBufferRef);
 
   if (OptionalObject)
     return isa<object::MachOObjectFile>(**OptionalObject)
@@ -885,6 +889,27 @@
 
   // squelch the error in case we had a non-object file
   consumeError(OptionalObject.takeError());
+
+  // If we're adding a bitcode file to the archive, detect the Archive kind
+  // based on the target triple.
+  LLVMContext Context;
+  if (identify_magic(MemBufferRef.getBuffer()) == file_magic::bitcode) {
+    auto ObjOrErr = object::SymbolicFile::createSymbolicFile(
+        MemBufferRef, file_magic::bitcode, &Context);
+    if (ObjOrErr) {
+      auto &IRObject = cast<object::IRObjectFile>(**ObjOrErr);
+      Triple TargetTriple(IRObject.getTargetTriple());
+      if (TargetTriple.isOSBinFormatMachO())
+        return object::Archive::K_DARWIN;
+      else if (TargetTriple.isOSBinFormatELF()) {
+        return TargetTriple.isOSBSDLike() ? object::Archive::K_BSD
+                                          : object::Archive::K_GNU;
+      }
+    }
+    // squelch the error in case this was not a SymbolicFile.
+    consumeError(ObjOrErr.takeError());
+  }
+
   return getDefaultForHost();
 }