The party has had a turbulent number of weeks as several party councillors have also quit the DUP recently.
Sinn Féin have more MLAs than the DUP for the first time in history following Stormont Assembly member Alex Easton's resignation from the party.
Easton cited a lack of "respect, discipline or decency" within the party as one of the reasons he is quitting, adding that he believes the UK government sold their "birth right" with the introduction of the protocol.
Easton announced his decision in a statement reported by the Co Down Spectator and Belfast Telegraph on Jeffrey Donaldson's first full day as DUP leader.
The resignation comes after two months of unprecedented internal turmoil for the party. The DUP has had a turbulent number of weeks as several party councillors have also quit the DUP recently.
"It is with great sadness and hurt that I find myself doing one of the hardest things in my life and resigning from the DUP," Easton's statement said.
"I have had to stand back and watch as colleagues tear themselves apart, brief against other colleagues and run to the media in order to hurt each other on a daily basis. 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 - it is not Alex Easton. No matter who people supported during the recent leadership contest, I find elements from both sides are equally to blame for recent events, and it continues."
The Stormont Assembly member also said the Westminster government are "destroying" the DUP with their implementation of the protocol.
"They have sold our birth right from under us with the Protocol, they just don't understand us. They will do anything it takes to appease the republican movement," he continued.
"Unionism is in my heart and soul, and I crave unity for the unionist community. I am not getting this from any of the unionist parties or their leaders at present. The Union is slowly being eroded from beneath our feet.
"We have a Westminster government who are destroying the DUP - through their actions over the head of unionism and with their implementation of the Protocol,"
With the DUP whip taken off Jim Wells and now Alex Easton's resignation, Sinn Fein now have more MLAs on the floor of the Assembly. It means a major shift for Northern Ireland's political landscape as it's the first time in history a Nationalist party has held the majority in the Assembly.
