Hope Hicks, one of President Trump's closest advisers, is to step down as White House communications director.
The announcement came a day after she testified in front of the House Intelligence Committee, but White House sources said this was not the reason.
Ms Hicks, 29, is reported to have told colleagues she was leaving because she felt she had achieved all she could. She has worked for Mr Trump for years.
She is the fourth person to serve as the president's communications chief.
On Tuesday, Ms Hicks gave a nine-hour testimony in front of the panel investigating alleged Russian meddling in the 2016 presidential election.
Ms Hicks is reported to have admitted to the panel that she had occasionally told what amounted to "white lies" for President Trump, but she denied lying about anything relevant to the investigation.
White House spokeswoman Sarah Sanders - who described Ms Hicks as "in a league of her own" - said it remained unclear when she would leave the administration.
There is no one that can fill the void Hope Hicks will leave behind. She is in a league of her own and no one can replace her. Far and away one of the most talented and skilled people I’ve ever met and coming to work won’t be the same without her.— Sarah Sanders (@PressSec) March 1, 2018
Ms Hicks is reported to have told colleagues she was leaving because she felt she had accomplished all she could in the White House.
Her departure will come as a major blow to Mr Trump, who said he would miss having her by his side. A former press secretary, she rose through the ranks to occupy the desk closest to the Oval Office.
Ms Hicks kept a low public profile but recently found herself part of a scandal when newspapers exposed that she was in a relationship with White House staff secretary Rob Porter, who had been accused of domestic violence against past partners.
When news of the allegations against Mr Porter broke last month, it was Ms Hicks who helped draft an initial White House statement defending him.
Mr Trump was reportedly not consulted and was unimpressed with her handling of the controversy.
Ms Hicks became head of the White House communications team last August, after the abrupt firing of Anthony Scaramucci.
In a statement, Mr Trump said: "Hope is outstanding and has done great work for the last three years. She is as smart and thoughtful as they come, a truly great person.
"I will miss having her by my side but when she approached me about pursuing other opportunities, I totally understood. I am sure we will work together again in the future."
Mr Scaramucci, speaking on Fox News, said: "She's one of the least malicious people I've ever met in my life. She's dedicated, she's charming, she's thoughtful, at the end of the day she's going to have an unbelievable career."
A former campaign official quoted by Politico magazine said Ms Hicks's departure left the White House without an expert "Trump translator".
"She knew what the president wanted and could explain it to the communications [team]," the official said.
Ms Hicks, Mr Scaramucci, Sean Spicer and Mike Dubke have all served in the role since January 2017.
The latest upheaval comes just after Jared Kushner, President Trump's son-in-law and senior adviser, had his White House security clearance downgraded.
On Tuesday night, the president also fired off a fierce tweet against Attorney General Jeff Sessions.
Why is A.G. Jeff Sessions asking the Inspector General to investigate potentially massive FISA abuse. Will take forever, has no prosecutorial power and already late with reports on Comey etc. Isn’t the I.G. an Obama guy? Why not use Justice Department lawyers? DISGRACEFUL!— Donald J. Trump (@realDonaldTrump) February 28, 2018
Out of the White House but likely not the spotlight
Analysis by the BBC's Anthony Zurcher in Washington
Hope Hicks had been there since the beginning; since before the beginning. When the Trump campaign was just a ragtag band of political neophytes, she was the one distributing press releases and answering media requests.
Where others had stumbled or been pushed out of Donald Trump's orbit, Hicks quietly persevered - and rode the train all the way to one of the most powerful White House jobs.
Now she too is gone. She lasted nearly as long as the preceding three White House communications directors combined, but the position continues to be cursed.
Administration sources insist that it was a planned exit, that she was simply waiting for the right time. It's hard, however, to imagine timing worse than this. It comes just a day after her eight hours of testimony before a congressional committee investigating possible Trump campaign ties to Russia, where she reportedly admitted to telling "white lies" in defence of the president.
Although she may be exiting the White House, it's unlikely she escapes the spotlight so easily. She had a ringside seat to many of the controversies that have swirled around the Trump campaign and presidency - and subsequent revelations could put her name in the headlines again.
Ms Hicks is seen as a key witness in the ongoing inquiry into whether the Trump team colluded with Russia.
During the nine-hour hearing on Tuesday, she reportedly stonewalled lawmakers about a 2016 meeting between members of the Trump campaign and a Russian lawyer at Trump Tower.
Democratic Senator Richard Blumenthal, a member of the powerful Senate Judiciary Committee investigating alleged Russian meddling, said that she now needed to testify.
"She has no right to claim executive privilege. She has no claim to refuse to come before the judiciary committee now she's stepped down. She should be a witness before the judiciary committee," he said.
Who is Hope Hicks?
- Brought up in Greenwich, Connecticut, and was a talented lacrosse player at high school and college
- Took up modelling as a teenager and once appeared in an ad for Ralph Lauren
- Previously worked for a public relations company that handled Ivanka Trump's fashion business and the Trump Organization's property brand
- Joined the Trump Organization in 2014 and Donald Trump brought her on to his campaign team a year later, despite her lack of political experience
- Nicknamed "Hopester" by Mr Trump, she is said to be one of his most trusted aides and among the few who could challenge him to change his views