Investigators said gunfire erupted around 6 p.m. as the bus was pulling up to the Puente Avenue stop on the westbound 10 Freeway.
The suspect and another man began arguing, leading to the suspect pulling out a gun and opening fire, according to the California Highway Patrol. Authorities said the bus driver tried to calm things down before the suspect pulled out the gun.
"The bus driver, from what we understand, did everything that he was trained to do by trying to diffuse the situation and mitigate it as best as possible," said CHP Lt. Saul Gomez. "Unfortunately, we have someone that was hurt as a result of this criminal."
A woman in her 50s was inadvertently struck and then rushed to a hospital with injuries described as not life-threatening, the CHP said. Authorities are still trying to determine if she was actually struck by a bullet or if she was injured by shrapnel, but she is expected to make a full recovery.
The second person involved in the fight was not injured.
Authorities believe the suspect fled to a nearby neighborhood. A suspect description was not immediately available, but investigators are reviewing surveillance cameras on the bus to help identify him.