According to the Pasadena Police Department, it happened Wednesday around 7:30 a.m. near the Metro Lake Avenue station.
The girl was sitting on a bus bench when the suspect sat next to her and sexually assaulted her, police said. The suspect, who was identified only as a man in his 30s or 40s, fled on a bicycle and remains at large.
Police said the girl reported the assault once she arrived at school.
A detailed description of the suspect wasn't immediately available but police say he was last seen wearing a long-sleeved jacket and black sweatpants.
Anyone with information is urged to contact the Pasadena Police Department.