The departure of Hicks, who worked as a one-woman communications shop during his campaign, came as a surprise to most in the White House - and cast a pall over the West Wing at a trying time for the president.
Hicks' resignation came a day after she testified before the House Intelligence Committee and said she had told white lies in the course of her duties, though there was no indication the two were connected. She told the House Intelligence Committee on Tuesday that she's told "white lies," but not on anything "substantive" for Trump.
In a statement, Mr. Trump commended her work and called her "outstanding". "I am sure we will work together again in the future", the president said.
White House Chief of Staff John Kelly said: "To say that she will be missed, is an understatement".
Ms. Hicks reportedly has been interviewed by the Senate Select Committee on Intelligence and special counsel Robert Mueller's team regarding the Russian Federation investigation.
Another trusted Trump adviser is departing the White House; reaction and analysis from the "Special Report" All-Star panel.
Last US summer, she was named communications director, overseeing a staff of dozens and responsible for the administration's overall communications strategy.
But from another angle, the top job in Trump's communications office was one that Hicks, a native of Greenwich, Connecticut, was born to do.
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During her appearance before the House Intelligence Committee, Trump's communications director had stuck to answering pre-approved questions, and refused to answer numerous other questions put to her.
She is expected to leave in few weeks, the White House said.
"Breaking news, White House in turmoil". The Hicks-Porter romance has reportedly since fizzled.
"And apparently that was the final straw for Hope Hicks".
Before joining Trump's presidential campaign in 2016, Hicks was an employee of the Trump Organization and has been long thought to be one of the president's closest advisers. Sources say she told the committee she occasionally told white lies, but never about Russian Federation. She never completely found her footing in Washington, and she was eager to return to the private sector, and continue her personal life with her family in CT.
It's unclear at this point who will be the next communications director.
Ms Hicks was Mr Trump's spokeswoman during the campaign.