Officers responded to Laguna and Adelaide streets around 5 p.m. and found the victim suffering the gunshot wound to the neck, Concord police Lt. Brian Wiesendanger said.
An investigation determined the man had been walking in the area when he saw another person being assaulted by a group, and attempted to stop the assault, he said.
One of the members of the group, believed to be a 16-year-old boy, shot the man, according to Wiesendanger. The group then fled on foot.
The victim of the shooting was treated and released for his injuries.
The alleged shooter is described as a black 16-year-old boy with a thin build. He was last seen wearing a green shirt, blue jeans, earrings and black shoes, Wiesendanger said.
Anyone with information about the shooting is asked to call the Concord Police Department crime tip hot line at (925) 603-5836.