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."

It was uncovered by archaeologists working in the villa's north range in Gloucestershire.
Other finds include two 4th Century Roman coins and a bone pin, and more of a mosaic which was first unearthed during a dig last year.
Experts said they had also found evidence of an earlier villa, beneath the 4th Century one.
National Trust archaeologist Martin Papworth said the tile was found in the changing rooms of the late Roman baths.
"The top tile that we came across had lots of imprints on it and we think that they're deer footprints.
"The tilers used to lay their tiles out to dry when they made them out of clay before they fired them and made them vulnerable to any animals walking around to put their prints on them.
"This looks like perhaps a Roe deer walked across and mucked up the surface of the tile, but it's quite nice to see it."
He said more of a mosaic, first unearthed last year, had been found.
"They had put concrete down in the 1960s to mark where walls had been found, and we expected there to be no mosaic there at all, but in the grassy islands between the concrete we found this amazing mosaic which had been missed in the 1960s.
"Now we know it was part of a great reception hall 18m (60ft) long by 6m (20ft) wide.  It's really exciting. It's patchy, but there are some large parts of it [remaining]."
He added that one of the 4th Century coins featured a Chi-Rho early Christian symbol, and was likely to be from the first Christian Roman emperor Constantine the Great.
A man targeted two teenagers waiting at a bus stop by sexually touching one and exposing himself in some nearby bushes.
[NEXT_CONCEPT]
A deer hoof print has been found on a 1,600 year-old roof tile during a dig at Chedworth Roman Villa.