Karen Bradley has been appointed as the new Northern Secretary.
It follows the resignation of James Brokenshire on health grounds.
She moves from her previous role as the UK Secretary for Digital, Culture, Media and Sport.
His decision came as British Prime Minister Theresa May announced a cabinet reshuffle.
The shake-up has merged health and social care into one political portfolio.
Jeremy Hunt remains as Heath Secretary in Theresa May's Cabinet, but has had social care added to his title.
Earlier, in a letter to Theresa May, James Brokenshire indicated that he is to undergo major surgery for a lesion on one of his lungs in the next few weeks.
He suggested he would "not be able to give the effort, energy and complete focus needed" as a result of the surgery - although stressed he hopes to return to work as an MP "at the earliest opportunity".
Mrs May responded: "You have undertaken some of the most important and demanding jobs in Government [...] with great diligence, determination and good humour.
The letters exchanged by James Brokenshire and Theresa May, as the Northern Ireland Secretary hands in his resignation due to ill health pic.twitter.com/Iu104FqE37
— Press Association (@PA) January 8, 2018
The move will be a blow to the British government as Brokenshire was a close ally of Mrs May.
Northern Ireland has been without a government for almost a year now, and Brokenshire had been heavily involved in talks aimed at re-establishing the Stormont executive.
He had first taken up the role in July 2016, following Mrs May's appointment as Prime Minister.
Our Political Correspondent Sean Defoe reports:
