The hit-and-run crash happened just after 8 a.m. as the officer was conducting a traffic stop on the right shoulder of the WB 91 Freeway near Euclid Street.
According to CHP, the officer was inside his vehicle when the driver of a black Nissan pickup truck smashed into it and fled. The truck was found under the Euclid Street overpass but the driver, who was described only as an adult male, remains at large.
Authorities said he was last seen wearing a white shirt and blue shorts.
The officer suffered only minor injuries.
Anyone with information is urged to contact CHP's Westminster office.