Arlene Foster has been elected as the first female party leader of the Democratic Unionist Party (DUP) by the party's MPs and assembly members.
The DUP's 90-member executive has ratified her appointment.
Mrs Foster replaces Peter Robinson, who announced his plans to step down as DUP leader and Northern Ireland first minister last month.
She said it was an "enormous honour and an even greater responsibility" to take up the role.
Mrs Foster was the only candidate for leader.
"It is truly humbling to follow in the footsteps of political giants like Ian Paisley and Peter Robinson," she added.
"For much of the last 40 years this party toiled in the political wilderness but today we stand tall as the largest unionist party and the party of Northern Ireland.
"That is down to the hard work and efforts of those who have gone before me.
"And as a result of that labour this role is not just as leader of the DUP but the leader of unionism.
"I want to build on the firm foundations that have been laid and take this party from strength to strength."
Prime Minister David Cameron and Northern Ireland Secretary of State Theresa Villiers said they looked forward to working with Mrs Foster "in building a brighter, more secure, future for everyone in Northern Ireland".
"I am sure that Arlene will be committed to doing the right thing for everyone in Northern Ireland," Ms Villiers said.
Sinn Féin's Martin McGuinness, who is Northern Ireland's Deputy First Minister, congratulated Mrs Foster.
"I will work positively with her for the benefit of all our people," he added.
Who is Arlene Foster?
Arlene Foster has experience of some of the most high-profile posts in Northern Ireland politics and has long been tipped for Stormont's top job.
The Fermanagh politician has had a rapid rise through the DUP ranks since joining the party from the Ulster Unionists in 2004.
She was born Arlene Kelly in Roslea in 1970.
Her first experience of Troubles violence came when she was just eight years old.
Her father was a part-time policeman and was shot by the IRA at the family farm.
When she was a teenager in 1988, a bomb exploded under her school bus.
Mrs Foster has represented the Fermanagh and South Tyrone constituency since 2003.
She is Stormont's current finance minister and has twice held the role of acting first minister of Northern Ireland, when Mr Robinson stepped aside temporarily in 2010 and in September this year.