The victim, Angie Hernandez, was with family and friends who were taking part in a Quinceanera, a Latino celebration for when a girl turns 15 years old, KGW-TV, Portland, reported.
Hernandez leaned against an emergency window about 6:30 p.m. Saturday and fell out of the bus, which was operated by Five Star Limousine, city police Lt. Robert King said.
King called it a tragic accident, the TV station said.
Rick Lycksell, general manager of Five Star Limousine, said the driver had a perfect record while working for the firm the past five years.
"He burst into tears, he was very upset ...," Lycksell said.