The Cornwall Councillor for Mabe, Perranarworthal and St Gluvias has resigned from his seat after just a year in post.
Michael Keogh, a member of UKIP, has cited personal reasons for his decision.
He said circumstances had arisen that meant he was unable to give the job the time it needed.
Mr Keogh, whose party saw huge success in last week’s European elections, was a member of the central sub-area planning committee.
He won his seat last May with a majority of just three votes.
A by-election is expected to be held in the coming months.