The former deputy leader of the Progressive Unionist Party has joined the UUP.
Belfast councillor John Kyle quit the PUP in December over "differing opinions" on Brexit's Northern Ireland Protocol.
The GP said he has been "impressed" with the direction in which Ulster Unionist leader Doug Beattie is taking the party.
He said: "I am very pleased to be joining the Ulster Unionist Party.
"I have been impressed with the direction in which Doug has been taking the party with a brand of positive, forward looking, inclusive unionism.
"That’s exactly what unionism should be.
"Doug’s vision of a prosperous, respectful and peaceful Northern Ireland in which everyone has a part to play is something that I want to be part of and work towards in the Ulster Unionist Party."
Mr Beattie said the pair "share the same vision for Northern Ireland".
He said: "I warmly welcome John to the Ulster Unionist Party. John is an impressive man of the utmost integrity who is respected right across the community and wider society.
"He is a committed unionist who is dedicated to promoting Northern Ireland, its people and its place within the United Kingdom.
"Through the many conversations I have had with him, I know we share the same vision for Northern Ireland and a forward-looking, confident unionism.
"I look forward to working with John in the days ahead. His level-headed approach to politics is refreshing and will be another great addition to the Ulster Unionist team."
Mr Kyle has served on Belfast City Council since 2007 when he was co-opted following the death of former PUP leader David Ervine.
Last year, Mr Kyle had spoken out about the Northern Ireland Protocol, arguing it had potential to offer "significant opportunities" if fundamental changes were made to it.
Most unionist representatives are fiercely opposed to the post-Brexit arrangements which include further checks on goods arriving into the region from Great Britain, regarding it as a border in the Irish Sea.
The PUP's policy is that the protocol should be scrapped.
In a statement last year the PUP said Mr Kyle had tendered his resignation.
"Recent media appearances have shown how his personal opinions were differing from party policy that we had laid out, but we had hoped that John would stay within the party and carry on the work for his community as a member of the Progressive Unionist Party," it said.
"Unfortunately that is not the case.
"Dr Kyle has been a valued member of the party for many years. Undoubtedly this is not a situation we welcome but we do wish John well and thank him for all his endeavours on behalf of our party and his service to the community."
His switch to the UUP increases the party's number of councillors in Belfast to four.
In December, Alliance Belfast councillor Carole Howard defected to the UUP, saying she could now "openly and comfortably" express her pro-Union views.
