Crews were called shortly after 1:30 p.m. to a homeless encampment in the Sepulveda Basin in Encino, where a grass fire had ignited.
LAFD says moments after first responders evacuated the area, a device by the encampment exploded.
Police later said what appeared to be a grenade was found at the scene, and a bomb squad was sent to the area to investigate. The cause of the fire remains under investigation.
LAFD officials said the injured firefighter was transported to Northridge Hospital in critical condition after he suffered severe burns to his face and ear.
His injuries were described as serious but not life threatening, and he was later released from the hospital.
The firefighter was seen walking before being airlifted to the hospital.
Ten other firefighters were evaluated for minor injuries.
Firefighters continued to douse flames from the fire hours after the explosion.