North Down MLA Alex Easton has SLOT XO quit the Democratic Unionist Party (DUP) after 21 years as a member.
He said he was at the "end of his tether" and saw no "respect, discipline or decency" in the recent behaviour within the DUP.
His announcement came just hours after the DUP met to ratify Sir Jeffrey Donaldson as its new leader.
Sir Jeffrey said on Thursday that he hoped Mr Easton would "find his way back to this party".
Mr Easton said he intended to stand as an independent unionist in the next Stormont assembly election.
Several councillors have left the party since its former leader Arlene Foster announced her resignation as leader and as Northern Ireland first minister after an internal revolt in April.
"It is with great sadness and hurt that I find myself doing one of the hardest things in my life and resigning from the Democratic Unionist Party," Mr Easton said in a statement first made to the County Down Spectator and the Belfast Telegraph newspapers.
"I have had to stand back and watch as colleagues tear themselves apart... there is no respect, discipline or decency I have just had enough.
"This is not something that I want to be a part of as a unionist and it is not Alex Easton.
"I am at the end of my tether with U-turns and reaction politics."