diff --git a/flang/lib/Optimizer/Dialect/FIRType.cpp b/flang/lib/Optimizer/Dialect/FIRType.cpp
--- a/flang/lib/Optimizer/Dialect/FIRType.cpp
+++ b/flang/lib/Optimizer/Dialect/FIRType.cpp
@@ -489,7 +489,9 @@
   ty = fir::unwrapRefType(ty);
   while (ty) {
     if (fir::isa_trivial(ty)) {
-      if (ty.isIntOrIndex()) {
+      if (mlir::isa<mlir::IndexType>(ty)) {
+        name << "idx";
+      } else if (ty.isIntOrIndex()) {
         name << 'i' << ty.getIntOrFloatBitWidth();
       } else if (ty.isa<mlir::FloatType>()) {
         name << 'f' << ty.getIntOrFloatBitWidth();
diff --git a/flang/unittests/Optimizer/FIRTypesTest.cpp b/flang/unittests/Optimizer/FIRTypesTest.cpp
--- a/flang/unittests/Optimizer/FIRTypesTest.cpp
+++ b/flang/unittests/Optimizer/FIRTypesTest.cpp
@@ -277,4 +277,6 @@
   mlir::Type ty = mlir::IntegerType::get(&context, 64);
   mlir::Type arrTy = fir::SequenceType::get({10, 20}, ty);
   EXPECT_EQ("10x20xi64", fir::getTypeAsString(arrTy, *kindMap));
+  EXPECT_EQ(
+      "idx", fir::getTypeAsString(mlir::IndexType::get(&context), *kindMap));
 }