Jeffrey Donaldson has said that he decided to leave the Ulster Unionists after a "vindictive'' meeting of the Ulster Unionist Executive last month.
“I have never been at a meeting like that meeting of the party executive that day,” he said.
“The level of vindictiveness that was displayed by some members to myself and my colleagues because of the stand we took in the election was something I have never experienced and I hope will never experience again.
“I make no apology for standing by the principles I believe in.”
The Lagan Valley MP said he intended to stick by the mandate he was given in the recent Assembly elections but felt that he could no longer do so within the Ulster Unionist Party.
The Democratic Unionists, he said, could fulfil that mandate on which he and his colleagues, Mrs Foster and Miss Beare, were elected to Stormont in November.
“Today we are announcing that we are joining the DUP. I am proud to be part of a team capable of providing leadership to the Unionist community – not like the leadership of the party I have left.
“Not like the leadership which has no bottom line, a leadership that does not know how to lead the Unionist community.
“I look forward to working with my new colleagues in the DUP and I will continue to represent the people of my constituency and to deliver for the people of Northern Ireland a better agreement.”