diff --git a/llvm/include/llvm/Object/ArchiveWriter.h b/llvm/include/llvm/Object/ArchiveWriter.h
--- a/llvm/include/llvm/Object/ArchiveWriter.h
+++ b/llvm/include/llvm/Object/ArchiveWriter.h
@@ -26,7 +26,6 @@
   sys::TimePoint<std::chrono::seconds> ModTime;
   unsigned UID = 0, GID = 0, Perms = 0644;
 
-  bool IsNew = false;
   NewArchiveMember() = default;
   NewArchiveMember(MemoryBufferRef BufRef);
 
diff --git a/llvm/lib/Object/ArchiveWriter.cpp b/llvm/lib/Object/ArchiveWriter.cpp
--- a/llvm/lib/Object/ArchiveWriter.cpp
+++ b/llvm/lib/Object/ArchiveWriter.cpp
@@ -48,7 +48,6 @@
     return BufOrErr.takeError();
 
   NewArchiveMember M;
-  assert(M.IsNew == false);
   M.Buf = MemoryBuffer::getMemBuffer(*BufOrErr, false);
   M.MemberName = M.Buf->getBufferIdentifier();
   if (!Deterministic) {
@@ -98,7 +97,6 @@
     return errorCodeToError(std::error_code(errno, std::generic_category()));
 
   NewArchiveMember M;
-  M.IsNew = true;
   M.Buf = std::move(*MemberBufferOrErr);
   M.MemberName = M.Buf->getBufferIdentifier();
   if (!Deterministic) {
@@ -236,13 +234,9 @@
 
 static void addToStringTable(raw_ostream &Out, StringRef ArcName,
                              const NewArchiveMember &M, bool Thin) {
-  StringRef ID = M.Buf->getBufferIdentifier();
-  if (Thin) {
-    if (M.IsNew)
-      Out << computeRelativePath(ArcName, ID);
-    else
-      Out << ID;
-  } else
+  if (Thin)
+    Out << computeRelativePath(ArcName, M.MemberName);
+  else
     Out << M.MemberName;
   Out << "/\n";
 }
@@ -254,7 +248,6 @@
                               StringRef ArcName, const NewArchiveMember &M,
                               sys::TimePoint<std::chrono::seconds> ModTime,
                               unsigned Size) {
-
   if (isBSDLike(Kind))
     return printBSDMemberHeader(Out, Pos, M.MemberName, ModTime, M.UID, M.GID,
                                 M.Perms, Size);
diff --git a/llvm/test/tools/llvm-ar/flatten-thin-archive-directories.test b/llvm/test/tools/llvm-ar/flatten-thin-archive-directories.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-ar/flatten-thin-archive-directories.test
@@ -0,0 +1,16 @@
+# This test creates a thin archive in a directory and adds it to a thin archive
+# in the parent directory. The relative path should be included when flattening
+# the archive.
+
+RUN: mkdir -p %t/foo
+RUN: touch %t/foo/a.txt
+RUN: rm -f %t/archive.a %t/foo/archive.a
+
+# These tests must be run in the same directory as %t/archive.a. cd %t is
+# included on each line to make debugging this test case easier.
+RUN: cd %t && llvm-ar rcST foo/archive.a foo/a.txt
+RUN: cd %t && llvm-ar rcST archive.a foo/archive.a
+RUN: cd %t && llvm-ar t archive.a | FileCheck %s --match-full-lines
+
+# FIXME: ar t mistakenly prepends / here, this should be "foo/a.txt".
+CHECK: /foo/a.txt
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
@@ -193,6 +193,9 @@
 // on the command line.
 static std::vector<StringRef> Members;
 
+// Static buffer to hold StringRefs.
+static BumpPtrAllocator Alloc;
+
 // Extract the member filename from the command line for the [relpos] argument
 // associated with a, b, and i modifiers
 static void getRelPos() {
@@ -544,6 +547,15 @@
   Expected<NewArchiveMember> NMOrErr =
       NewArchiveMember::getOldMember(M, Deterministic);
   failIfError(NMOrErr.takeError());
+  if (FlattenArchive) {
+    // The archive child member we're trying to flatten is relative to the
+    // archive it's in, so use the full path here and it will be added as
+    // relative to the archive we're adding it to.
+    StringSaver Saver(Alloc);
+    Expected<std::string> FileNameOrErr = M.getFullName();
+    failIfError(FileNameOrErr.takeError());
+    NMOrErr->MemberName = Saver.save(*FileNameOrErr);
+  }
   if (FlattenArchive &&
       identify_magic(NMOrErr->Buf->getBuffer()) == file_magic::archive) {
     Expected<std::string> FileNameOrErr = M.getFullName();
@@ -580,8 +592,11 @@
       return;
     }
   }
-  // Use the basename of the object path for the member name.
-  NMOrErr->MemberName = sys::path::filename(NMOrErr->MemberName);
+  // For regular archives, use the basename of the object path for the member
+  // name. For thin archives, relative paths will be resolved when writing
+  // members, so leave it as the full path here.
+  if (!Thin)
+    NMOrErr->MemberName = sys::path::filename(NMOrErr->MemberName);
   Members.push_back(std::move(*NMOrErr));
 }
 
@@ -671,7 +686,7 @@
           computeInsertAction(Operation, Child, Name, MemberI);
       switch (Action) {
       case IA_AddOldMember:
-        addChildMember(Ret, Child);
+        addChildMember(Ret, Child, /*FlattenArchive=*/Thin);
         break;
       case IA_AddNewMember:
         addMember(Ret, *MemberI);
@@ -679,7 +694,7 @@
       case IA_Delete:
         break;
       case IA_MoveOldMember:
-        addChildMember(Moved, Child);
+        addChildMember(Moved, Child, /*FlattenArchive=*/Thin);
         break;
       case IA_MoveNewMember:
         addMember(Moved, *MemberI);
@@ -950,7 +965,6 @@
 
 static int ar_main(int argc, char **argv) {
   SmallVector<const char *, 0> Argv(argv, argv + argc);
-  BumpPtrAllocator Alloc;
   StringSaver Saver(Alloc);
   cl::ExpandResponseFiles(Saver, cl::TokenizeGNUCommandLine, Argv);
   for (size_t i = 1; i < Argv.size(); ++i) {