Girl indecently assaulted on Sydney bus

A 16-year-old school girl has been indecently assaulted by a passenger and followed to school after giving up her seat on a bus in Sydney's west.

The girl boarded a bus on Wilmot Street in St Marys about 10.15am on Wednesday, but vacated her seat for an elderly passenger, after which a man seated nearby also left his seat and stood behind her.

Police say the man threatened the girl and indecently assaulted her a short time later.

The victim got off the bus on Phillip Street, but the man followed her past St Marys railway station and to her school.

The man, described as being of Caucasian appearance, about 40-50 years old, about 183cm tall with a black/grey coloured beard and of medium build, remained outside for some time before leaving, police said.

He was last seen wearing black tracksuit pants and a grey hooded jumper.

Anyone with any information is asked to contact Crime Stoppers.