diff --git a/mlir/lib/IR/BuiltinAttributes.cpp b/mlir/lib/IR/BuiltinAttributes.cpp
--- a/mlir/lib/IR/BuiltinAttributes.cpp
+++ b/mlir/lib/IR/BuiltinAttributes.cpp
@@ -302,6 +302,19 @@
                                  Type type) {
   if (!Dialect::isValidNamespace(dialect.strref()))
     return emitError() << "invalid dialect namespace '" << dialect << "'";
+
+  // Check that the dialect is actually registered.
+  MLIRContext *context = dialect.getContext();
+  if (!context->allowsUnregisteredDialects() &&
+      !context->getLoadedDialect(dialect.strref())) {
+    return emitError()
+           << "#" << dialect << "<\"" << attrData << "\"> : " << type
+           << " attribute created with unregistered dialect. If this is "
+              "intended, please call allowUnregisteredDialects() on the "
+              "MLIRContext, or use -allow-unregistered-dialect with "
+              "mlir-opt";
+  }
+
   return success();
 }
 
diff --git a/mlir/lib/IR/BuiltinTypes.cpp b/mlir/lib/IR/BuiltinTypes.cpp
--- a/mlir/lib/IR/BuiltinTypes.cpp
+++ b/mlir/lib/IR/BuiltinTypes.cpp
@@ -201,6 +201,19 @@
                                  Identifier dialect, StringRef typeData) {
   if (!Dialect::isValidNamespace(dialect.strref()))
     return emitError() << "invalid dialect namespace '" << dialect << "'";
+
+  // Check that the dialect is actually registered.
+  MLIRContext *context = dialect.getContext();
+  if (!context->allowsUnregisteredDialects() &&
+      !context->getLoadedDialect(dialect.strref())) {
+    return emitError()
+           << "`!" << dialect << "<\"" << typeData << "\">"
+           << "` type created with unregistered dialect. If this is "
+              "intended, please call allowUnregisteredDialects() on the "
+              "MLIRContext, or use -allow-unregistered-dialect with "
+              "mlir-opt";
+  }
+
   return success();
 }
 
diff --git a/mlir/test/IR/attribute.mlir b/mlir/test/IR/attribute.mlir
--- a/mlir/test/IR/attribute.mlir
+++ b/mlir/test/IR/attribute.mlir
@@ -1,4 +1,4 @@
-// RUN: mlir-opt %s -split-input-file -verify-diagnostics | FileCheck %s
+// RUN: mlir-opt %s -split-input-file -allow-unregistered-dialect -verify-diagnostics | FileCheck %s
 
 //===----------------------------------------------------------------------===//
 // Test integer attributes
diff --git a/mlir/test/IR/invalid-unregistered.mlir b/mlir/test/IR/invalid-unregistered.mlir
new file mode 100644
--- /dev/null
+++ b/mlir/test/IR/invalid-unregistered.mlir
@@ -0,0 +1,14 @@
+// RUN: mlir-opt %s -split-input-file -verify-diagnostics
+
+// expected-error @below {{op created with unregistered dialect}}
+"unregistered_dialect.op"() : () -> ()
+
+// -----
+
+// expected-error @below {{attribute created with unregistered dialect}}
+#attr = #unregistered_dialect.attribute
+
+// -----
+
+// expected-error @below {{type created with unregistered dialect}}
+!type = type !unregistered_dialect.type
diff --git a/mlir/unittests/IR/AttributeTest.cpp b/mlir/unittests/IR/AttributeTest.cpp
--- a/mlir/unittests/IR/AttributeTest.cpp
+++ b/mlir/unittests/IR/AttributeTest.cpp
@@ -150,6 +150,7 @@
 
 TEST(DenseSplatTest, StringSplat) {
   MLIRContext context;
+  context.allowUnregisteredDialects();
   Type stringType =
       OpaqueType::get(Identifier::get("test", &context), "string");
   StringRef value = "test-string";
@@ -158,6 +159,7 @@
 
 TEST(DenseSplatTest, StringAttrSplat) {
   MLIRContext context;
+  context.allowUnregisteredDialects();
   Type stringType =
       OpaqueType::get(Identifier::get("test", &context), "string");
   Attribute stringAttr = StringAttr::get("test-string", stringType);