White House Press Secretary Sean Spicer has quit, reportedly in protest at an organisational shake-up.
Mr Spicer stepped down because he was unhappy with President Donald Trump's appointment of a new communications director, according to reports.
Combative Wall Street financier Anthony Scaramucci has been picked for the role that Mr Spicer partially filled.
Mr Spicer's press briefings were a cable news hit, but in recent weeks he withdrew from the camera's glare.
He told US media that the White House "could benefit from a clean slate".
Later, appearing on Fox News, Mr Spicer said that with Mr Scaramucci's appointment there was a risk of "too many cooks in the kitchen" in the White House media operation, which was why he was stepping down.
The 45 year old said he had told Mr Trump he "would stay on for a few weeks to achieve a smooth transition", and was looking forward to spending more time with his family.
He defended the president's agenda and hit out at what he termed "media bias", saying: "I was increasingly disappointed about the way the media here do their job, or don't do their job."
The shake-up at the White House comes amid several investigations into alleged Russian meddling in last year's US presidential election and whether Mr Trump's campaign team colluded with Moscow.
The New York Times reported that Mr Spicer had "vehemently" disagreed with the appointment of Mr Scaramucci, which he believed to be a "major mistake".
Spicer's low points
- inflating crowd size estimates at Trump inauguration at first briefing
- his appearance, particularly his suits, reportedly criticised by Trump
- saying Hitler never used chemical weapons and referring to Holocaust "centres"
- butt of text message joke by adviser Steve Bannon about his weight
- defending Trump "covfefe" tweet by saying it had hidden meaning
- frozen out of meeting with the Pope in Rome, despite being devout Catholic
- not invited to Paris for Trump visit
Announcing that he would step down next month, Mr Spicer tweeted that it had been "an honor & a privilege to serve" the president.
The BBC's White House reporter Tara McKelvey says journalists besieged the 18-acre complex so they could film Mr Spicer leaving.
What does Scaramucci say?
In an assured debut, he attended Friday afternoon's news conference to announce that Sarah Huckabee Sanders, formerly Mr Spicer's deputy, would step into his shoes.
"I love the president and it's an honour to be here," Mr Scaramucci said, adding: "He is genuinely a wonderful human being."
He also said: "The president has really good karma."
Mr Scaramucci, who has no previous experience in communications roles, paid tribute to Mr Spicer as "a true American patriot" and "incredibly gracious".
"I hope he goes on to make a tremendous amount of money," he said.
Mr Scaramucci also apologised and said he had been "unexperienced" as he explained his previous criticism of the president.
In an August 2015 interview with Fox Business, he dismissed Mr Trump as a "hack" and "an inherited money dude" with "a big mouth".
Mr Scaramucci is currently senior vice-president of the Export-Import Bank, a US government agency which guarantees loans for foreign buyers of American exports.
A former member of the Trump transition team, he mistakenly suggested to the BBC in January that Elton John would play at the new president's inauguration.
The singer promptly denied it.
Bigger tremors to come? Analysis by Anthony Zurcher, BBC News
Life on a White House staff is intense and exhausting.
This administration is under particular pressure, given the ongoing Russia investigation, recent legislative setbacks and a president who can be, shall we say, occasionally off-message. Now cracks in the structure are beginning to show.
Sean Spicer's departure, reportedly because he doesn't want to work for newly named communication director Anthony Scaramucci, represents the most significant shakeup within the administration's senior team to date. It could also be a sign of bigger tremors to come.
Mr Spicer was closely allied with White House chief of staff Reince Priebus, who was his boss last year in the Republican National Committee.
The Trump team has been rife with personal feuds, as various factions vie for a mercurial president's ear. Leaks abound.
The prestige and power of advisers and aides wax and wane. Media reports abound of a White House under siege.
And just a reminder - the Trump presidency is only six months old.
Ms Huckabee Sanders read a statement from President Trump, saying: "I am grateful for Sean's work on behalf of my administration and the American people.
"I wish him continued success as he moves on to pursue new opportunities. Just look as his great television ratings."
On day one in January, Mr Spicer set the tone of his relationship with the press by bursting into the briefing room to berate journalists for their reporting of crowd numbers at President Trump's inauguration.
His proclivity for gaffes and garbling of his words, as well as making debatable assertions, soon made Mr Spicer a household name.
But he could also be charming and was liked by many among the press corps.
Mr Spicer was lampooned on topical comedy show Saturday Night Live, where Melissa McCarthy played him as a loud-mouthed bully who brandished his lectern at reporters.
Mr Trump noted approvingly in April that Mr Spicer "gets great ratings". A month later, the president said: "He's doing a good job but he gets beat up."
Mr Spicer was roundly mocked after he reportedly hid by a hedgerow on the White House grounds to avoid reporters on the night Mr Trump fired the FBI director in May.
His last on-camera briefing was on 20 June, and there have been few since then.
Members of the media have accused the Trump administration of attempting to kill off the daily news conferences to avoid scrutiny.