Percy Jackson's father being Poseidon is true. In the Percy Jackson book series, it is revealed that Percy is a demigod, meaning he is the son of a god and a mortal. His godly parent is Poseidon, the Greek god of the sea. Throughout the series, Percy's connection to Poseidon is explored, and he possesses powers and abilities related to water and the sea. This relationship is a central aspect of the story and plays a significant role in Percy's adventures.