At about 11:40 a.m., police were dispatched to a report of a shooting in the 500 block of Norcross Lane near Oakley Elementary School.
Officers arrived at the scene and found a man on the sidewalk in front of a residence.
The 23-year-old man was suffering from a gunshot wound and was pronounced deceased by paramedics at the scene, police said.
The incident is being investigated as a homicide. No motive has been established yet and the identity of the deceased man is being withheld pending notification of his next of kin, according to police.
Anyone with any information about the homicide is asked to call the Contra Costa County Sheriff's Office at (925) 646-2441.
Tips may be sent via an anonymous voice message to (866) 846-3592.