Paula Bradshaw, who represents South Belfast in the Assembly, said 10 years ago she dressed up as the black American actor.
Mr T is best known for playing the character BA Baracus in the 1980s television series The A-Team.
Ms Bradshaw posted an apology on Twitter on Saturday, describing it as a “serious lapse of judgement”.
It has come to my attention that there are photos of me on social media from a decade ago dressed as Mr. T.
For some time I have been deeply embarrassed I ever thought such an inappropriate costume was a good idea and I apologise for any offence caused.
— Paula Bradshaw MLA (@PaulaJaneB) September 19, 2020
“For some time I have been deeply embarrassed I ever thought such an inappropriate costume was a good idea and I apologise for any offence caused,” she tweeted.
“It was a serious lapse of judgement. It did not reflect my views then and does not reflect them now.
“I am proud to stand on my record of tackling the very real issues of racial discrimination and prejudice faced by minority ethnic communities here and abroad every day.
“I will not be diverted from continuing to take concrete action to support minority communities, assist refugees and oppose racial injustice in all its forms.”