The incident occurred at around 1.45pm on Thursday on Glen Luss Road in Luss, Argyll and Bute, near the junction with the A82.
A 19-year-old woman was approached by a group of three men and one of them pushed her to the ground between some bushes.
He then put his hand down her top, but the woman was able to push the man off and run away.
She was not physically injured during the incident.
Detective Sergeant Peter Jarvis of Clydebank CID said: “This was very scary and distressing incident for the woman involved and she has been left quite shaken.
“We are eager to trace the man responsible as soon as possible and I would appeal to anyone who may recognise the description of the suspect, or who may have seen the three men in the area around this time, to get in touch.
“It is thought the men may have got into a parked car on the Glen Luss Road, near to the Faerie Trail.”
The suspect is described as a white man in his early 20s, around 5ft 8in tall and of skinny build.
At the time of the incident he was wearing blue shorts with white trainers and was in the company of two other men also in their early 20s.
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