The party had been at a residence near the intersection of West Ninth and I streets, police said.
Police responded to reports of a shooting nearby at 2:42 a.m.
Two men were found there suffering from gunshot wounds. One of the victims died at the scene, police said.
The deceased victim's name has not been released pending notification of his next of kin.
Anyone with information about the shooting as been asked to call Antioch police Detective Desmond Bittner at (925) 779-6939.