Antioch police received a call from the victim at about 8:20 a.m. reporting that he had been shot in the area of Somersville Road and Delta Fair Boulevard.
When officers got there, they found the victim sitting in his pickup truck with a gunshot wound to his back.
The victim and his passenger told police that they had been stopped at a red light on Somersville Road at Delta Fair Boulevard when a white four-door sedan pulled up behind them, police said.
When the light turned green, the white car went around them on the left and then both vehicles made a right turn onto Delta Fair Boulevard.
As they were making the turn, the driver of the white car fired a shot at the victim. The bullet went through the driver's side door and struck the victim in the back.
The white car, which was occupied by a male driver and a female passenger, then fled west on Delta Fair Boulevard.
The victim was taken to a local hospital where he was being treated for his injury, which is serious but not believed to be life-threatening, police said.
Anyone with information about the case is asked to call the Antioch Police Department's investigations bureau at (925) 779-6926.