Grattheos or Gratios, the God of the Sea (海神グラッテオス), is a foreign deity to the land of Altago and part of the religion of Tritheism. Grattheos was adopted by the Tribe of Edonas about 200 years before the start of Ys Seven. The Edonians believed that by worshipping Grattheos, they could gain a higher level of prosperity.

Grattheos is currently being worshipped mostly by people living in the City of Altago who are mostly descendants of Edonians. The Edonians had moved from the Ruins Island and forced the Iskans off their land where the current capital of the kingdom is located. The establishment of Grattheos saw an increase level of commercialism and wealth compared to the remaining Five Tribes of Altago.

A statue of Grattheos can be seen on top of the Dragon Knights' naval base that is situated in the harbor of Altago. Other memorabilia of Grattheos includes the Grattheos' talisman which can allow the user to breath underwater indefinitely.

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