Hope Hicks, the White House communications director and one of President Trump's longest serving advisers, is to step down, the administration says.
The 29-year-old former model 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 White House communications chief.
White House spokeswoman Sarah Sanders said it remained unclear when Ms Hicks would leave the administration.
She said her resignation was not connected to testimony she gave to the House Intelligence Committee on Tuesday.
She 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.
Before him, 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 was close to the president - famously so - and aloof with reporters, according to BBC White House Reporter Tara McKelvey. During conversations on Air Force One or on the tarmac, Ms Hicks was polite but cool, she says.
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.
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.