Officers arrived in the area of Vine Street and Mitchell Lane at about 1:20 am to find a man with multiple gunshot wounds.
They tried to revive the man but he died at the scene.
Homicide detectives are canvassing the area for evidence and possible surveillance video.
They are looking for a red sedan reported to have been leaving the area at the time.
If you have any information about the case, you are urged to contact the Atwater Police Department at 209-357-6384 and ask for Detective Sgt Brum or Detective Vargas.
This is Atwater's third homicide of the year, police say.