Hope Hicks is resigning from her job as White House communications director, White House spokesperson Sarah Sanders has said.
Ms Hicks is one of US President Donald Trump's longest-serving aides.
Ms Sanders told reporters the timeline for Ms Hicks' departure was unclear.
She added that the departure was not related to her testimony to a congressional committee yesterday about a probe into Russian meddling in the 2016 presidential election.
She is expected to leave in the next few weeks.
Ms Hicks, 29, was caught up in a controversy surrounding former White House staff secretary Rob Porter, whom she had been dating.
She worked to defend him when charges of domestic abuse against his two former wives emerged. Mr Porter was ultimately forced to resign.
She was one of the first people hired by Mr Trump when he began his campaign for the presidency, and is one of his most trusted advisers.
Aides said she had approached the president and told him she wanted to leave so she could start exploring opportunities outside of the White House.
"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 released by the White House.
"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."
Yesterday Ms Hicks declined to answer questions about the administration from members of the US House of Representatives Intelligence Committee investigating Russia and the 2016 election, politicians said.
She is reported to have admitted telling "white lies" for Mr Trump in her role as his spokeswoman during the campaign.
She took over as communications director in September after the short, difficult tenure of Anthony Scaramucci, who was fired.
She is credited behind the scenes for stabilising the communications operation.
Ms Hicks said in a statement: "There are no words to adequately express my gratitude to President Trump. I wish the President and his administration the very best as he continues to lead our country."