The shooting happened around 6:15 p.m. near the interchange of the 8 and 805 freeways in San Diego's Mission Valley area. Officials said the officer and the suspect became involved in a struggle when the suspect pulled out a gun and shot him. Social media posts showed bystanders holding down the suspect and tending to the officer.
The investigation prompted freeway closures in both directions, according to CalTrans.
During a press conference, CHP Captain Michael Harris the eight-year veteran officer was investigating a crash when the shooting occurred. The name of the officer was not released.
CHP said the officer suffered a gunshot wound to his right thigh and was in serious, but stable condition at Scripps Mercy Hospital.
San Diego police have reported that the shooter is in custody.
No further information was released.