The crash happened near 4th Street and Hermosa Avenue around 8 p.m.
Investigators say Rancho Cucamonga deputies were responding to a report of an armed robbery in the 9000 block of Hermosa Avenue when the crash occurred.
Jodi Miller with the San Bernardino County Sheriff's Department says the deputy was traveling in the No. 2 lane on Hermosa Avenue when a semi truck pulled in front of him to make a right-hand turn into a driveway. Miller says that's when the deputy's patrol car collided with the truck.
The deputy was hospitalized in serious condition, but authorities say he did not suffer life-threatening injuries.
The driver of the big rig was not injured. He was questioned by authorities but not placed under arrest.
Investigators are still looking into the crash.