Rob Curtis was given a two year conditional discharge following the incident on Skomer Island off Pembrokeshire in June 2014.
He is a member of Vale of Glamorgan council and Barry town council.
The case was referred to the Adjudication Panel for Wales which said Mr Curtis failed to comply with the code of conduct of both councils.
Mr Curtis is a Labour member representing the Gibbonsdown ward for both councils.
He was a member of the county council's cabinet until last year.
The Adjudication Panel met on 25 July this year and was told Mr Curtis, who was 50 at the time, touched the leg of a teenage girl while working as a volunteer on Skomer.
The panel decided unanimously that he failed to comply with the code of conduct which states councillors should not bring their role or their local authority into disrepute.
The tribunal's written ruling, published this week, said Mr Curtis' conduct "was unacceptable".
"The resulting criminal proceedings and the negative publicity surrounding those proceedings could be regarded as bringing the office of member into disrepute," it added.
It said that the suspension was necessary "in order to uphold public confidence in democratic institutions and maintain proper standards of conduct".
The ruling added that Mr Curtis had the right to appeal to the High Court over the decision.
A town and county councillor has been suspended after admitting common assault on a 17-year-old girl.