Republican representative Elise Stefanik is a close Trump ally and was chosen by the president for the UN role less than a week after he was elected in November.
CBS News first reported that Stefanik's nomination could be withdrawn.
"It is essential that we maintain EVERY Republican Seat in Congress," Trump said in a social media post.
"With a very tight Majority, I don't want to take a chance on anyone else running for Elise's seat."
Republicans hold a narrow 218-213 majority in the US House of Representatives, where there are four vacancies, as they prepare to try to extend Trump's 2017 tax cuts and address a debt that tops $US36.6 trillion ($A58 trillion).
Stefanik was the last cabinet-level Trump nominee who had not been confirmed.
She was approved by the Senate Foreign Relations Committee, with some Democratic votes, on January 30 and had been expected to be easily approved by the full Senate.
US House Speaker Mike Johnson praised Stefanik.
"It is well known Republicans have a razor-thin House majority, and Elise's agreement to withdraw her nomination will allow us to keep one of the toughest, most resolute members of our Conference in place to help drive forward President Trump's America First policies," he said in a social media post.
Veteran US diplomat Dorothy Shea has been the acting US ambassador to the UN since former president Joe Biden's envoy Linda Thomas-Greenfield stepped down in January.
It was not immediately clear who Trump might choose to replace Stefanik.
"There are others that can do a good job at the United Nations," Trump wrote.
Trump said in February that the United Nations has "great potential and ... we'll continue to go along with it but they got to get their act together".
The UN pushed back at the time, saying Secretary-General Antonio Guterres had worked tirelessly to implement reforms.
Earlier this month Guterres announced a new initiative to improve efficiency and cut costs as the world body turns 80 this year amid a cash crisis.
