diff --git a/mlir/lib/IR/BuiltinTypes.cpp b/mlir/lib/IR/BuiltinTypes.cpp
index 786c30851a071..0bebbe7aea33a 100644
--- a/mlir/lib/IR/BuiltinTypes.cpp
+++ b/mlir/lib/IR/BuiltinTypes.cpp
@@ -616,6 +616,12 @@ bool mlir::detail::isSupportedMemorySpace(Attribute memorySpace) {
   if (llvm::isa<IntegerAttr, StringAttr, DictionaryAttr>(memorySpace))
     return true;
 
+  // Allow opaque attributes if unregistered dialects are allowed.
+  // They hold unregistered custom dialect attributes.
+  if (memorySpace.getContext()->allowsUnregisteredDialects() &&
+      isa<OpaqueAttr>(memorySpace))
+    return true;
+
   // Allow custom dialect attributes.
   if (!isa<BuiltinDialect>(memorySpace.getDialect()))
     return true;
diff --git a/mlir/test/IR/parser.mlir b/mlir/test/IR/parser.mlir
index 3bb6e38b4d613..a22f788288e5d 100644
--- a/mlir/test/IR/parser.mlir
+++ b/mlir/test/IR/parser.mlir
@@ -127,6 +127,12 @@ func.func private @memrefs_nomap_dictspace(memref<5x6x7xf32, {memSpace = "specia
 // CHECK: func private @memrefs_map_dictspace(memref<5x6x7xf32, #map{{[0-9]*}}, {memSpace = "special", subIndex = 3 : i64}>)
 func.func private @memrefs_map_dictspace(memref<5x6x7xf32, #map3, {memSpace = "special", subIndex = 3}>)
 
+// CHECK: func private @memrefs_nomap_opaquespace(memref<5x6x7xf32, #unknown_dialect.unknown_attr>)
+func.func private @memrefs_nomap_opaquespace(memref<5x6x7xf32, #unknown_dialect.unknown_attr>)
+
+// CHECK: func private @memrefs_map_opaquespace(memref<5x6x7xf32, #map{{[0-9]*}}, #unknown_dialect.unknown_attr>)
+func.func private @memrefs_map_opaquespace(memref<5x6x7xf32, #map3, #unknown_dialect.unknown_attr>)
+
 // CHECK: func private @complex_types(complex<i1>) -> complex<f32>
 func.func private @complex_types(complex<i1>) -> complex<f32>