Differences between the books and the films

Character's Ages
Instead of being twelve years old, Percy, Annabeth, and Clarisse appear to be seventeen.

Prophecies
The Prophecy has significant changes, such as the child of the Big Three (Zeus, Poseidon and Hades) would either preserve or raise Olympus at the age of 20 rather than 16.

Characters

 * Grover in the film is portrayed with having dark skin.
 * Hades is portrayed as malovant and cruel. Persephone is mentioned to have been held prisoner in the underworld and that she hates her husband.
 * Annabeth in the first film had brown hair and blue eyes. However, in the second film she has blond hair because Alexandra Daddario dyed her hair blonde for The Sea of Monsters.

Character Exemptions

 * Circe (mentioned instead)
 * Tantalus (mentioned instead)

Locations

 * The underworld is portrayed as a hell like appearance rather than having all of its locations just like in Greek mythology