He claims there is no room for independent or logical thinking within the party.
Lord Maginnis decided to resign after senior party members, including leader Mike Nesbitt, distanced themselves from him after he described gay marriage as "unnatural and deviant behaviour" in a radio interview.
Afterwards, Lord Maginnis stood by his comments in a follow-up interview with a Belfast newspaper which resulted in party leader Mike Nesbitt removing the party whip from him.
Responding to today's announcement, Mr. Nesbitt said Lord Maginnis was the sort of progressive Unionist he admired.
"I hope we can now focus on a legacy of service over a number of decades rather than concentrate on more recent matters, which I believe could have been resolved at any time," he added.