Ms Haley would not confirm the report to Reuters when asked about it during a visit to the White House on Tuesday.
White House spokeswoman Sarah Sanders said Trump and Haley were scheduled to meet in the Oval Office at 10:30 a.m. (1430 GMT).
Donald Trump tweeted: “Big announcement with my friend Ambassador Nikki Haley in the Oval Office at 10:30am.”
White House press secretary Sarah Sanders tweeted: President Trump and Ambassador Nikki Haley will meet in the Oval Office at 10:30am this morning.
“This event will be open to the pool”.
Trump accepted the resignation and Ms Haley discussed her resignation with Trump last week when she visited him at the White House, Axios reported, citing two sources.
CNN confirmed the resignation, Axios was the first to report it.
So far 26 people have either been sacked or resigned from Trump’s administration.
More to follow…