The officer suffered facial injuries and was transported to an area hospital. His condition was not known.
Officials said the driver of the car made an unsafe left turn on Roscoe Boulevard at Willis Avenue and crashed into the officer.
Roscoe Boulevard was closed between Van Nuys and Sepulveda boulevards following the accident. Commuters were advised to use Parthenia Street or Sherman Way as alternate routes.
It was unknown if the driver of the car, who was not injured, was detained.