The Los Angeles County Sheriff's Department identified the suspect as 59-year-old Majdeddin Nahvi, a transient living in Canoga Park.
Sheriff's investigators say the 16-year-old victim did not report the first two assaults because she thought she would never see the suspect again. They say after the third assault, the teen took a cellphone photo of Nahvi as he left the bus and then called authorities.
Deputies say the suspect would take a seat next to the teen, cover himself with a sweater and touch her. She was not hurt in any of the incidents, which took place between Aug. 27 and Sept. 30.
Nahvi was arrested on Tuesday and booked into the L.A. County Jail. His bail was set at $100,000.
Anyone with information on the case was asked to contact L.A. Crime Stoppers at 1-800-222-TIPS.