Nikki Haley suddenly resigns as Trump's UN ambassador
Meeting Haley in the Oval Office, Trump said she had done a "fantastic job" and would stay on to the end of the year.
"She told me probably six months ago," Trump told reporters, that '"I want to take a little time off.'"
A successor will be named "in the next two or three weeks," he said.
However, Haley was coy on her reasons for quitting, saying only that it was "important to understand when it's time to stand aside" after a string of challenging jobs.
US media reports quoted administration officials saying privately that Haley's departure was a surprise even to Secretary of State Mike Pompeo.
In a statement to journalists at the White House that lasted a few seconds, Pompeo called her a "great partner." He then refused to take questions.
US media speculated that Haley has ambitions for running against Trump in the primaries ahead of the 2020 elections, where she might be seen as a more moderate Republican alternative to his turbulent and often divisive presidency.
But she quashed the rumor. "No I'm not running," she said.
Trump and Haley went out of their way at the Oval Office meeting to emphasize that the resignation was taking place on amicable terms -- something far from always the case in the turbulent first two years of the administration.
But while Washington insiders questioned whether Haley's departure signaled a new round of palace intrigue, a more prosaic explanation for the move was that she needs to earn a serious private sector salary.
Indeed, her 2018 government ethics financial disclosure shows her with huge debt, including a mortgage of more than $1 million.