Castillo Nunatak is a conspicuous rocky peak rising to 437 m on the glacial divide between Huntress Glacier and Balkan Snowfield on Hurd Peninsula in eastern Livingston Island in the South Shetland Islands, Antarctica. It is linked to Charrúa Ridge to the west Charrúa Gap and, via Willan Nunatak and Willan Saddle, to Burdick Ridge to the east-northeast. Castillo Nunatak is a prominent landmark in the Charrúa Gap area.
The feature is descriptively named for its abrupt and isolated appearance ('Castle Nunatak' in Spanish).