The DUP's Willie McCrea has defeated the UUP's David Burnside in South Antrim.
Willie McCrea wins South Antrim from the UUP
The gap was bigger than had been expected between the two men with Mr McCrea taking 14,507 votes to Mr Burnside's 11,059.
The UUP defeat is the fourth loss it has suffered - the biggest being the defeat of David Trimble as leader.
Mr McCrea said: "I believe this is a red letter day for unionism. The people of Ulster have spoken loud and clear."
The South Antrim seat has see-sawed between Mr McCrea and Mr Burnside in recent years. Mr McCrea won the seat in a by-election in 2000.
However, in the last general election, Mr Burnside regained the seat by a majority of 1,011 votes for the Ulster Unionists.
During the election campaign Mr Burnside said he thought the DUP and UUP should merge when David Trimble and Ian Paisley leave politics.
"There will be a realignment and I would prefer a merger," he said.
"There is no point in having two small groupings in Westminster."