Pirate's Grotto is a location that appears in Paper Mario: The Thousand-Year Door. It is where the fifth Crystal Star is located and the boss that lurks here is Cortez, the Pirate King. It is located on Keelhaul Key.
Pirate's Grotto is mainly a large, cave area that has many twists and turns. Another notable feature is the water where Mario can learn the Paper Boat transformation from the fourth and final Black Chest Demon. There is also areas where old, broken ships can be seen.
Mario and company head into the Pirate's Grotto where they look for the next Crystal Star. However, a creepy voice keeps telling Mario to turn back or will suffer a fate. Mario ignores the messages each time and treads further and further into the area. Eventually, Mario reaches an area where Toads can be seen stranded on a single boat. Mario manages to free them and, to his surprise, meets up with Francesca Pianta and Frankie again. They say that they are vacationing on Keelhaul Key but got stranded on a boat. Mario then heads further where he reaches a boat known as the Black Skull and soon finds himself fighting the King of Pirates: Cortez. Cortez thinks Mario is after his treasure and fights him in battle. Mario and his friends manage to defeat Cortez in battle but Cortez isn't fully gone since he is already dead. However, Mario tells Cortez that he just wants his Crystal Star and Cortez gladly gives it up. Mario earns the fifth Crystal Star and learns the Sweet Feast move. Mario then heads out and uses Bobbery's ability to blow up a way out to the sea.
- The Pirate's Grotto is similar in a way to Dry Dry Ruins from Paper Mario as they are both areas explored by Mario where he hears cryptic messages while exploring the area.