"She wears masks a lot but she tested positive and I just went out with a test and the first Lady just went out with a test also. In the meantime, we will begin our quarantine process!"
Trump later tweeted that he and the first lady will begin isolating until they receive their results.
Hicks returned to the White House earlier this year after a stint in the private sector.
At 74, Trump is at higher risk of serious complications from the virus.
Over the years, she has developed a reputation for knowing and understanding him better than most advisers.
Trump's handling of the pandemic has already been a major flashpoint in his race against Biden, who spent much of the summer off the campaign trail and at his home in DE citing concern about the virus.
Hicks, 31, traveled with Trump to Cleveland for his first debate with Biden on Tuesday. "When needed, I wear masks". "When needed, I wear masks".
The White House released the president's Friday schedule earlier in the evening, confirming that he plans to travel to Florida for a campaign rally.
Updated 1:30 a.m.: Dow futures and S&P futures are off 1.5%, and Nasdaq futures have slid 1.8%.
In a Fox News interview late Thursday night, Trump appeared unsure about what his top aide's positive test meant for him.
What Hope Hicks' positive COVID-19 test means for Trump
Hope Hicks , who serves as counselor to the president, became the latest Trump aide to come down with the highly contagious virus. White House aides are tested for the virus regularly; some, like White House chief of staff Meadows, are tested daily.
An administration official anonymously revealed to the Associated Press that Hicks tested positive after traveling with Trump to a rally on Wednesday.
He also called Hicks a "warm" person with military members and "it's very hard to say 'stay back'". The AP also reported Trump's children and senior staff violated host venue rules by not wearing masks during the debate.
President Donald Trump announced early Friday that he and his wife both tested positive for the coronavirus, an extraordinary development coming months into a global pandemic and in the final stretch of his reelection campaign in which he has flouted experts' guidance on preventing the disease's spread.
A letter released by the White House said the couple "both feel well at this time".
In Britain, chief scientific adviser Patrick Vallance told reporters "things are definitely heading in the wrong direction" as the government extended lockdowns to several towns in northern England, effectively putting more than a quarter of the country under tighter restrictions.
The news was sure to rattle an already shaken nation still grappling with how to safely reopen the economy without driving virus transmission.
Melania Trump tweeted that she and her husband were "feeling good" but have canceled their upcoming public events.
In a statement, Judd Deere, the Deputy White House Press Secretary said that the President takes the health and safety of himself and everyone who works in support of him and the American people very seriously.
French authorities said they may place Paris on maximum virus alert as soon as Monday, potentially requiring all bars to close as the number of cases surges.
She was seen in a photo on Wednesday not wearing a mask after exiting Marine One with Kushner and White House aide Stephen Miller.
"On behalf of the Israeli people, our thoughts and wishes are with President Trump and the First Lady for a refuah shlema, a swift and full recovery", Rivlin wrote.