Ryan McCready, who sits on Derry City and Strabane District Council, initially left to become an independent unionist.
He said the DUP was “no longer compatible” with his beliefs.
Mr McCready, 35, is a former soldier who served in Iraq and Afghanistan.
He was one of a number of DUP councillors who left the party following the removal of Arlene Foster.
She was replaced by Edwin Poots, who announced after 21 days that he would resign and was replaced by Sir Jeffrey Donaldson.
‘Impressive individual’
At the time, Mr McCready said he “took serious issue” with the manner in which Mrs Foster “was ousted”.
“If that’s the way we treat people I don’t want to be part of it,” he added.
The councillor said the “tipping point” for his decision to leave the party was the brief leadership of Mr Poots.
On Monday, UUP leader Doug Beattie, who was a captain in the Royal Irish Regiment, welcomed Mr McCready to his party by describing him as “impressive individual and impressive councillor”.
“I have known him for many, many years, and I am absolutely delighted he has decided the vision the Ulster Unionist Party has for the future is a vision he can share,” he told BBC Radio Foyle.
“I see Ryan as somebody who will be on the benches in Stormont.
“There are processes to go through but I hope Ryan will now put his name forward in order to stand in the next assembly election. He is really someone who can add real value to the party.”
Darren Guy, the UUP’s group leader on Derry City and Strabane District Council, said Mr McCready would be a “welcome addition” to the party.
“We look forward to working with Ryan and believe his politics are best suited to the progressive politics of the Ulster Unionist Party,” he said.
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