diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -5250,15 +5250,22 @@
   if (!TemplateParamsOrErr)
     return TemplateParamsOrErr.takeError();
 
-  // FIXME: Import default argument.
-
   TemplateTemplateParmDecl *ToD = nullptr;
-  (void)GetImportedOrCreateDecl(
-      ToD, D, Importer.getToContext(),
-      Importer.getToContext().getTranslationUnitDecl(), *LocationOrErr,
-      D->getDepth(), D->getPosition(), D->isParameterPack(),
-      (*NameOrErr).getAsIdentifierInfo(),
-      *TemplateParamsOrErr);
+  if (GetImportedOrCreateDecl(
+          ToD, D, Importer.getToContext(),
+          Importer.getToContext().getTranslationUnitDecl(), *LocationOrErr,
+          D->getDepth(), D->getPosition(), D->isParameterPack(),
+          (*NameOrErr).getAsIdentifierInfo(), *TemplateParamsOrErr))
+    return ToD;
+
+  if (D->hasDefaultArgument()) {
+    Expected<TemplateArgumentLoc> ToDefaultArgOrErr =
+        import(D->getDefaultArgument());
+    if (!ToDefaultArgOrErr)
+      return ToDefaultArgOrErr.takeError();
+    ToD->setDefaultArgument(Importer.getToContext(), *ToDefaultArgOrErr);
+  }
+
   return ToD;
 }
 
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -901,6 +901,39 @@
           .isValid());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       TemplateTemplateParmDeclNoDefaultArg) {
+  Decl *FromTU = getTuDecl(R"(
+                           template<template<typename> typename TT> struct Y {};
+                           )",
+                           Lang_CXX17);
+  auto From = FirstDeclMatcher<TemplateTemplateParmDecl>().match(
+      FromTU, templateTemplateParmDecl(hasName("TT")));
+  TemplateTemplateParmDecl *To = Import(From, Lang_CXX17);
+  ASSERT_FALSE(To->hasDefaultArgument());
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase, TemplateTemplateParmDeclDefaultArg) {
+  Decl *FromTU = getTuDecl(R"(
+                           template<typename T> struct X {};
+                           template<template<typename> typename TT = X> struct Y {};
+                           )",
+                           Lang_CXX17);
+  auto From = FirstDeclMatcher<TemplateTemplateParmDecl>().match(
+      FromTU, templateTemplateParmDecl(hasName("TT")));
+  TemplateTemplateParmDecl *To = Import(From, Lang_CXX17);
+  ASSERT_TRUE(To->hasDefaultArgument());
+  const TemplateArgument &ToDefaultArg = To->getDefaultArgument().getArgument();
+  ASSERT_TRUE(To->isTemplateDecl());
+  TemplateDecl *ToTemplate = ToDefaultArg.getAsTemplate().getAsTemplateDecl();
+
+  // Find the default argument template 'X' in the AST and compare it against
+  // the default argument we got.
+  auto ToExpectedDecl = FirstDeclMatcher<ClassTemplateDecl>().match(
+      To->getTranslationUnitDecl(), classTemplateDecl(hasName("X")));
+  ASSERT_EQ(ToTemplate, ToExpectedDecl);
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase,
        ImportOfTemplatedDeclOfClassTemplateDecl) {
   Decl *FromTU = getTuDecl("template<class X> struct S{};", Lang_CXX03);