Limavady councillor Aaron Callan has quit the Ulster Unionist Party.
In a short statement the party confirmed his departure.
It said: "We have built our success in recent years on teamwork. On that basis Councillor Callan is unlikely to be missed."
Mr Callan was elected to Causeway Coast and Glens Borough Council in 2014.
A senior associate with PricewaterhouseCoopers and Queen's law graduate, he is also a keen historian.
The 29-year-old is also Deputy Master of Bellarena Orange Lodge.
At the Ulster Unionist Party conference in 2013 Mr Callan spoke of his pride in being a member of the party.
On helping to build a prosperous economy he said it was "the right thing to do for Northern Ireland".
"And that's why I am proud to say I am an Ulster Unionist," he said.