Trevor Lunn has resigned from the Alliance Party and will become an independent MLA, the party has announced.
A spokesperson said they were sorry to learn of the Lagan Valley MLA's resignation and that the reason for it remained unclear after a meeting with Alliance leader Naomi Long.
Mr Lunn (73) has been MLA for Lagan Valley since 2007 and is a former mayor of Lisburn.
Late last year, he apologised after video footage was posted on social media appearing to show him slapping a dog.
He previously made headlines for backing gay marriage after having voiced his opposition for a period of time.
In July 2018, he paid tribute to the NHS after a stay in hospital.
"He had said for some time he intended to stand down from politics at the next election. It is sad he has chosen to end his political career outside the Alliance family," the party spokesperson said.
"We recognise his contribution to the party. We wish him well for the future."
Mr Lunn said the move was a "sad decision for me", which was caused by "internal difficulties".