Surrey Police said two teenage girls were waiting at a bus stop half way along Rydens Road, in Walton-on-Thames, when a man approached them and touched one of the girls on the bottom.
He then went into some nearby bushes and exposed himself.
He is described as white, in his mid 20s and about 6ft tall. He had short ginger or blonde hair and wonky teeth.
The incident happened at 19:30 BST on Thursday.
Det Con Ben Kirby said: "Prior to this incident, the girls had been travelling on the 461 bus to Walton town centre. However, it had broken down so they were waiting for another bus to continue their journey.
"We would like to speak to anyone who was on this bus as they might have information which will help us to identify this individual."
A man targeted two teenagers waiting at a bus stop by sexually touching one and exposing himself in some nearby bushes.