US Attorney General Jeff Sessions has resigned as the country's chief law enforcement officer after months of criticism from President Trump.
Mr Sessions announced he would resign at the president's request in a letter to the White House on Wednesday following the midterm elections.
Donald Trump announced on Twitter that Mr Sessions' chief of staff Matt Whitaker would become the new acting attorney general.
He said: "We thank Attorney General Jeff Sessions for his service, and wish him well!"
The resignation was the culmination of a toxic relationship that frayed just weeks into Mr Sessions' tumultuous tenure, when he stepped aside from the investigation into potential co-ordination between the president's campaign and Russia.
Mr Trump blamed the decision for opening the door to the appointment of special counsel Robert Mueller, who took over the Russia investigation and began examining whether Mr Trump's hectoring of Mr Sessions was part of a broader effort to obstruct justice and stymie the probe.
Senate minority leader Chuck Schumer said the timing of the resignation is "very suspect" and a new attorney general must not interfere with the Mueller probe.
He added the president would create a "constitutional crisis" if he were to impede the investigation into possible Russian collusion in the presidential election.
Mr Trump's outspoken stance on Mr Sessions earned the president accusations that he was trying to interfere in the legal system of the United States.
In September, the president said he does not "have an attorney general" in a fierce attack on Mr Sessions. He also criticised him for his stance on immigration.
In August, Mr Sessions said: "While I am attorney general, the actions of the Department of Justice will not be improperly influenced by political considerations."
President Trump had raised the prospect of firing Mr Sessions as far back as September.
Asked whether he would consider firing Mr Sessions in an interview with political website Hill.TV, he said: "We'll see what happens. A lot of people have asked me to do that."
The relentless attacks came despite the fact the Alabama Republican was the first US senator to endorse Mr Trump and his crime-fighting agenda and priorities - particularly his hawkish immigration enforcement policies - largely mirrored the president's.
