Paul Givan said holding the office of First Minister was the "privilege of his life" as he announced his resignation on Thursday.
The DUP MLA is stepping down from the position with effect from midnight, as part of his party’s opposition to the Northern Ireland Protocol.
It means Sinn Féin’s Michelle O’Neill will also cease to be deputy First Minister, as the two offices are joined as part of Northern Ireland's power-sharing government.
However, the move does not automatically trigger the collapse of the Stormont institutions.
“Today marks the end of what has been the privilege of my lifetime to serve as the First Minister of Northern Ireland,” said Mr Givan.
“Holding this office is one that comes with a heavy responsibility, and I have often felt the weight of this burden to do what is right for all our people.
“Working together as a five party Executive has its challenges but over the past eight months, working with the deputy First Minister Michelle O’Neill, we made progress and have made a difference that will improve our society."
The move has been widely criticised by the other parties as a "manufactured crisis" and a "gross betrayal of people in Northern Ireland".
Secretary of State Brandon Lewis described it as "extremely disappointing" and said he would urge the DUP to "reinstate the First Minister immediately to ensure the necessary delivery of public services for the citizens of Northern Ireland".
Sinn Féin has called for an early Assembly election to take place.
Ms O’Neill said: “It’s very clear that there are catastrophic impacts in terms of the DUP’s action today and there are many casualties as a result of their political opportunism."
Political parties react to the news.
Mr Givan was appointed as First Minister last June, taking over from then-DUP leader Arlene Foster who was ousted following a leadership challenge.
The Lagan Valley MLA became the youngest holder of the post aged 39, and his tenure will be the shortest in Northern Ireland's history.
Judith Hill looks back at the stormy recent history of devolution.
It comes as the DUP continues its protest against the implementation of the NI Protocol.
Party leader and Stormont agriculture minister, Edwin Poots, instructed officials to stop carrying out agri-food checks at ports from midnight.
However, there has been no confirmation from Stormont officials whether they intend to comply with the order.
DUP Leader Sir Jeffrey Donaldson described the protocol as "economic madness".
Sir Jeffrey Donaldson outlines the DUP's position.
He said: "The Government now accepts the harm that the protocol is doing and appears to accept that not only is change required but that it must be secured if the gains made as a result of recent political progress are not to be squandered.
"In the 217 days that I have been leader of this party, the Northern Ireland Protocol has cost our economy £535m. That's £2.5m every single day.
"And over £100,000 every single hour."