The shooting happened just before 2 a.m. near Division Street and avenue 33, according to Los Angeles police.
The off-duty officer was attending a family party and heard shots fired before he or she went outside and saw a person who had been shot in the leg by someone in a vehicle. The officer then fired at the car and the suspect took off.
It was not yet unknown whether anyone in the car was struck by gunfire.
Police said they were interviewing witnesses at the scene.