Edwin Poots was elected the new leader of Northern Ireland's Democratic Unionist Party on Friday.
Mr Poots, 55, defeated Sir Jeffrey Donaldson in the race to replace outgoing leader and first minister Arlene Foster.
The election comes amid unionist anger over the installation of a post-Brexit customs border with the rest of the UK.
Speaking after his victory was announced, Mr Poots said Brexit's Northern Ireland Protocol was a "massive challenge for us".
“If we are to fight this, to ensure that everybody in Northern Ireland is not worse off as a consequence of the protocol then it is for us to do that together,” he said.
Mr Poots, who won 19 out of 36 ballots in Friday’s contest, plans to nominate another party member to replace Ms Foster as first minister.
She faced the wrath of party members for backing the Brexit agreement, which was intended to avoid a hard border on the island of Ireland.
Anger over the post-Brexit rules was a contributing factor to a week of street violence in Northern Irish cities last month.
It raised fears of a revival of The Troubles, the decades-long sectarian conflict in the region in which 3,500 people were killed.
Mr Poots is a traditionalist-leaning agriculture minister long known as an opponent of the protocol.
He called on Friday for an end to bickering in the unionist movement and said he would reach out to other unionist leaders.
“This party has been the authentic voice of unionism and will continue to be the authentic voice of unionism under my leadership,” he said.
“I will be a leader in unionism who will be reaching out to other leaders in unionism.”
One of the other parties, the Ulster Unionist Party, is also set to hold a leadership election after leader Steve Aiken resigned on Saturday.
The power-sharing Northern Ireland Assembly is balanced between unionists and Irish nationalists under the terms of the 1998 peace deal which ended The Troubles.
