Jeffrey Donaldson has announced he will be a candidate in the contest to succeed Arlene Foster as DUP leader.
He's currently the party's leader at Westminster.
He wrote to MPs and members of Northern Ireland's assembly to confirm he's entering the race, ahead of publicly announcing his bid on Monday morning.
He'll be up against Stormont agriculture minister Edwin Poots, who is also running for the leadership.
Mr Donaldson said: "Our previous leaders have shown that when the DUP leads, Northern Ireland moves forward.
"We are the custodians of the union. Let us take our next steps forward in Northern Ireland."
He was speaking on the centenary of the creation of Northern Ireland, with Mr Donaldson today suggesting Northern Ireland's "best days are ahead of us".
Edwin Poots confirmed he was running for DUP leader last week.
He suggested over the weekend that he'd split the role of party leader from Northern Ireland first minister if he wins the leadership contest.
Arlene Foster is due to step down as DUP leader at the end of May, and as first minister a month later.
Her decision to step down came after a majority of the party's MLAs backed a letter calling on her to resign.