Causeway Coast and Glens councillor Cathal McLaughlin, of Mounthill Park in Cloughmills, was convicted at trial in Scotland under section three of the Sexual Offences Scotland Act (2009) on July 11.
The offence in question took place in Scotland on October 15, 2021.
In a statement, a Sinn Fein spokesperson said they had written to McLaughlin to inform him of his expulsion.
“Sinn Fein has written to Councillor Cathal McLaughlin upon receipt of information that he has been convicted in a Scottish court of a serious criminal offence,” the spokesperson said.
“Our thoughts are with the victim of this serious crime of sexual assault.
“Cathal McLaughlin failed to inform the party at any stage that he was subject to criminal proceedings.
“Given the severity of the offence, his membership of Sinn Fein is terminated with immediate effect.”
McLaughlin previously served on the old Ballymoney Borough Council before being co-opted on to Causeway Coast and Glens Borough Council in 2016 to replace Philip McGuigan, following his election to the Assembly.
At the time of his 2016 co-option, McLaughlin said: "It is a honour to be selected to represent the people of North Antrim as a councillor.
"I will aim to build on the good work of Philip McGuigan and work for the people of North Antrim and across the constituency."
Upon his co-option to the council to serve the Ballymoney district electoral area, McLaughlin said his priorities would include developing a campaign to reduce speed limits in areas where there are vulnerable people and children, and also addressing the “disparity” of the clearing of roads in the winter in Ballymoney, compared to other areas.
McLaughlin is due to be sentenced on August 8.
Speaking to the Belfast Telegraph, McLaughlin said: "I am totally innocent of what I have been accused of and will be appealing. I didn't inform the party of the case as I was assured nothing would come of it.”
Causeway Coast ands Glens Borough Council have been contacted for comment. McLaughlin’s contact page on the council’s website has been removed.