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
@@ -345,7 +345,8 @@
     Or<!foreach(allowedtype, allowedTypes, allowedtype.predicate)>,
     !if(!eq(summary, ""),
         !interleave(!foreach(t, allowedTypes, t.summary), " or "),
-        summary)>;
+        summary),
+    cppClassName>;
 
 // Integer types.
 
diff --git a/mlir/include/mlir/TableGen/Type.h b/mlir/include/mlir/TableGen/Type.h
--- a/mlir/include/mlir/TableGen/Type.h
+++ b/mlir/include/mlir/TableGen/Type.h
@@ -49,7 +49,7 @@
   Optional<StringRef> getBuilderCall() const;
 
   // Return the C++ class name for this type (which may just be ::mlir::Type).
-  StringRef getCPPClassName() const;
+  std::string getCPPClassName() const;
 };
 
 // Wrapper class with helper methods for accessing Types defined in TableGen.
diff --git a/mlir/lib/TableGen/Type.cpp b/mlir/lib/TableGen/Type.cpp
--- a/mlir/lib/TableGen/Type.cpp
+++ b/mlir/lib/TableGen/Type.cpp
@@ -11,6 +11,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "mlir/TableGen/Type.h"
+#include "mlir/TableGen/Dialect.h"
+#include "llvm/ADT/Twine.h"
 #include "llvm/ADT/TypeSwitch.h"
 #include "llvm/TableGen/Record.h"
 
@@ -54,8 +56,19 @@
 }
 
 // Return the C++ class name for this type (which may just be ::mlir::Type).
-StringRef TypeConstraint::getCPPClassName() const {
-  return def->getValueAsString("cppClassName");
+std::string TypeConstraint::getCPPClassName() const {
+  StringRef className = def->getValueAsString("cppClassName");
+
+  // If the class name is already namespace resolved, use it.
+  if (className.contains("::"))
+    return className.str();
+
+  // Otherwise, check to see if there is a namespace from a dialect to prepend.
+  if (const llvm::RecordVal *value = def->getValue("dialect")) {
+    Dialect dialect(cast<const llvm::DefInit>(value->getValue())->getDef());
+    return (dialect.getCppNamespace() + "::" + className).str();
+  }
+  return className.str();
 }
 
 Type::Type(const llvm::Record *record) : TypeConstraint(record) {}
diff --git a/mlir/test/mlir-tblgen/op-decl.td b/mlir/test/mlir-tblgen/op-decl.td
--- a/mlir/test/mlir-tblgen/op-decl.td
+++ b/mlir/test/mlir-tblgen/op-decl.td
@@ -247,6 +247,23 @@
 // CHECK: static void build(::mlir::OpBuilder &, ::mlir::OperationState &odsState, ::mlir::TypeRange resultTypes, ::mlir::ValueRange operands, ::llvm::ArrayRef<::mlir::NamedAttribute> attributes = {});
 // CHECK: static void build(::mlir::OpBuilder &odsBuilder, ::mlir::OperationState &odsState, ::mlir::ValueRange operands, ::llvm::ArrayRef<::mlir::NamedAttribute> attributes = {});
 
+// Test that type defs have the proper namespaces when used as a constraint.
+// ---
+
+def Test_Dialect2 : Dialect {
+  let name = "test";
+  let cppNamespace = "::mlir::dialect2";
+}
+def TestDialect2Type : TypeDef<Test_Dialect2, "Dialect2Type">;
+
+def NS_ResultWithDialectTypeOp : NS_Op<"op_with_dialect_type", []> {
+  let results = (outs TestDialect2Type);
+}
+
+// CHECK-LABEL: NS::ResultWithDialectTypeOp declarations
+// CHECK:     class ResultWithDialectTypeOp :
+// CHECK-SAME: ::mlir::OpTrait::OneTypedResult<::mlir::dialect2::Dialect2TypeType>
+
 // Check that default builders can be suppressed.
 // ---