diff --git a/mlir/include/mlir/IR/OpBase.td b/mlir/include/mlir/IR/OpBase.td
--- a/mlir/include/mlir/IR/OpBase.td
+++ b/mlir/include/mlir/IR/OpBase.td
@@ -2861,6 +2861,7 @@
       AttrOrTypeDef<"Attr", name, traits, baseCppClass> {
   // The name of the C++ Attribute class.
   string cppClassName = name # "Attr";
+  let storageType = dialect.cppNamespace # "::" # name # "Attr";
 
   // The underlying C++ value type
   let returnType = dialect.cppNamespace # "::" # cppClassName;
diff --git a/mlir/test/mlir-tblgen/op-attribute.td b/mlir/test/mlir-tblgen/op-attribute.td
--- a/mlir/test/mlir-tblgen/op-attribute.td
+++ b/mlir/test/mlir-tblgen/op-attribute.td
@@ -6,6 +6,7 @@
 
 def Test_Dialect : Dialect {
   let name = "test";
+  let cppNamespace = "foobar";
 }
 class NS_Op<string mnemonic, list<OpTrait> traits> :
     Op<Test_Dialect, mnemonic, traits>;
@@ -17,6 +18,10 @@
   let constBuilderCall = "some-const-builder-call($_builder, $0)";
 }
 
+def SomeAttrDef : AttrDef<Test_Dialect, "SomeAttr"> {
+}
+
+
 // Test required, optional, default-valued attributes
 // ---
 
@@ -271,6 +276,19 @@
 // DECL-LABEL: EOp declarations
 // DECL: static void build({{.*}}, uint32_t i32_attr, uint32_t dv_i32_attr, ::llvm::APFloat f64_attr, ::llvm::APFloat dv_f64_attr, ::llvm::StringRef str_attr, ::llvm::StringRef dv_str_attr, bool bool_attr, bool dv_bool_attr, ::SomeI32Enum enum_attr, ::SomeI32Enum dv_enum_attr = ::SomeI32Enum::case5)
 
+
+// Test proper namespacing for AttrDef
+// ---
+
+def NamespaceOp : NS_Op<"namespace_op", []> {
+  let arguments = (ins
+      SomeAttrDef:$AttrDef
+  );
+}
+// DECL: NamespaceOp
+// DECL: foobar::SomeAttrAttr AttrDef()
+
+
 // Test mixing operands and attributes in arbitrary order
 // ---