Hope Hicks, one of President Trump’s longest-serving advisers, is to step down as White House communications director.
The 29-year-old former model and ex-Trump Organization employee has been by Mr Trump’s side for years.
She is reported to have told colleagues she felt she had accomplished all she could in the White House.
She is the fourth person to serve as communications chief for this administration.
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.
Ms Sanders said the resignation was not connected to testimony Ms Hicks gave to the House Intelligence Committee on Tuesday.
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 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 into alleged Russian meddling in the 2016 election, US media reported.
During the election campaign, Ms Hicks served as press secretary. She took over as the head of the White House communications team last August, after the abrupt firing of Anthony Scaramucci.
Mr Scaramucci, speaking on Fox News, described her as “a wonderful person”.
“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,” he said.
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.
Before Mr Scaramucci, Sean Spicer and Mike Dubke both served in the role – which involves overseeing a busy press department.
But Ms Hicks has kept a remarkably low profile in the job.
“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,” Mr Trump said in a statement.
“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.”
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.
Earlier this month her actions were scrutinised amid a scandal involving White House staff secretary Rob Porter, who quit amid domestic abuse allegations. She was, at the time, reported to have been dating Mr Porter.
Ms Hicks helped draft an initial statement defending Mr Porter and her handling of the controversy reportedly displeased Mr Trump.