Xerxes (クセルクセス kuserukusesu) (spelled 'Cselkcess' in Viz manga) was an ancient country that existed in the Great Desert to the east of Amestris four hundred years before the start of the series. Despite having had a remarkably advanced society for their era, the entire population of Xerxes was mysteriously wiped out in a single night, leaving only ruins behind.

Xerxes was a relatively peaceful, thriving land despite the harsh conditions of the desert that surrounded it and, at the time of its sudden disappearance, had a population of over a million people. Xerxes appears to have had a markedly intellectual culture that fostered written language as well as philosophical and alchemical studies. Though slavery seems to have been a common practice, it was possible for slaves to rise out of thralldom through education, even going so far as to become members of high society. Xerxes was also a centralized monarchy, with a King and royal court governing from the capital, a castle city in the center of the country. For the most part, Xerxesian people are characterized by having light complexions as well as gold-colored hair and eyes; however, there does seem to have been a minority of citizens with darker skin tones and similarly light-colored hair.

In the current continuity, the ruins of Xerxes consist mainly of the decaying remnants of the king's castle and capital city; the famous, yet remote site acts as an oasis for travelers venturing across the Great Desert from Amestris to Xing or vice-versa. Additionally, the ruins serve as a haven in which a number of Ishvalan refugees have taken refuge in the years after the Ishval Civil War.