diff --git a/src/demangle.cc b/src/demangle.cc
index d7b3af1..dd26914 100644
--- a/src/demangle.cc
+++ b/src/demangle.cc
@@ -1097,10 +1097,11 @@ static bool ParseTemplateArgs(State *state) {
 // <template-arg>  ::= <type>
 //                 ::= <expr-primary>
 //                 ::= I <template-arg>* E        # argument pack
+//                 ::= J <template-arg>* E        # argument pack
 //                 ::= X <expression> E
 static bool ParseTemplateArg(State *state) {
   State copy = *state;
-  if (ParseOneCharToken(state, 'I') &&
+  if ((ParseOneCharToken(state, 'I') || ParseOneCharToken(state, 'J')) &&
       ZeroOrMore(ParseTemplateArg, state) &&
       ParseOneCharToken(state, 'E')) {
     return true;
diff --git a/src/demangle_unittest.txt b/src/demangle_unittest.txt
index 4e23c65..fd6aca0 100644
--- a/src/demangle_unittest.txt
+++ b/src/demangle_unittest.txt
@@ -135,3 +135,7 @@ _ZlsRSoRKSs	operator<<()
 _ZngILi42EEvN1AIXplT_Li2EEE1TE	operator-<>()
 _ZplR1XS0_	operator+()
 _Zrm1XS_	operator%()
+
+# Template argument packs can start with I or J.
+_Z3addIIiEEvDpT_	add<>()
+_Z3addIJiEEvDpT_	add<>()