Snellville police identified the injured officer as Jermarcus McCoy. He has been an officer for seven months, according to the department.
He is recovering and expected to be released soon, police say.
Two other people injured in the wreck suffered some serious, but non-life-threatening injuries.
Police shut down Scenic Highway in both directions between Wisteria Drive and Dogwood Road during the investigation.
Detective Corporal Jeff Manley with Snellville PD said a driver pulled out in front of a squad car responding to a call while its sirens and lights were active.
"The civilian driver was ejected from the vehicle and did sustain injuries and was taken to a local hospital," Manley said. "The officer also sustained injuries and was taken to the hospital."
Georgia State Patrol is investigating and said the civilian driver was not wearing a seatbelt.
Investigators said McCoy was responding to an overturned vehicle accident at around 2:30 p.m.
Troopers said McCoy traveled into the center turn lane, which had heavy traffic. The other driver, in a Ford Focus, pulled out in front of McCoy from a shopping strip.
Investigators said McCoy tried to swerve left to avoid the collision but collided with the other car. Snellville police said a green passenger vehicle was also involved in the collision.
McCoy and both other drivers were transported to Northside Gwinnett Hospital.
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