Mr Trump tweeted: “Hope Hicks, who has been working so hard without even taking a small break, has just tested positive for Covid 19. Terrible!
“The First Lady and I are waiting for our test results. In the meantime, we will begin our quarantine process!”
Ms Hicks who serves as counsellor to the president and travelled with him to a rally on Wednesday, tested positive on Thursday.
The 31-year-old is the closest aide to Mr Trump to test positive so far.
Prior to tweeting, the president told Fox News’ Sean Hannity that he and first lady Melania Trump were awaiting their own test results, adding that he did not know “whether we have it”.
It can take days for an infection to be detectable by a test. It was not immediately clear what Mr Trump’s quarantine entailed, but The Associated Press cited the president as saying he expected to have his test results back either on Thursday night or Friday morning.
Mr Trump is 74 years old, putting him at higher risk of serious complications from the virus.
Minutes before his tweet, the White House distributed a schedule for Friday that showed he planned to go forward with a fundraiser at his Washington DC hotel and a political rally in Sanford, Florida.
The positive test is yet another reminder that the coronavirus continues to spread, even as Mr Trump has tried desperately to suggest it no longer poses a danger.
Since it emerged earlier this year, Mr Trump, the White House and his campaign have played down the threat and refused to abide by basic public health guidelines — including those issued by his own administration — such as wearing masks in public and practicing social distancing.
Instead, Mr Trump has continued to hold campaign rallies that draw thousands of supporters.
The virus has killed more than 200,000 Americans and infected more than seven million nationwide.
Ms Hicks travelled with the president multiple times this week, including aboard Marine One, the presidential helicopter, for a Minnesota rally on Wednesday, and aboard Air Force One to Tuesday night’s first presidential debate against Joe Biden.
Multiple White House staffers have tested positive for the virus, including Katie Miller, Vice President Mike Pence’s press secretary, as well as one of the president’s personal valets.
In a statement, White House spokesman Judd Deere said: “The president takes the health and safety of himself and everyone who works in support of him and the American people very seriously.”
“White House Operations collaborates with the physician to the president and the White House Military Office to ensure all plans and procedures incorporate current CDC guidance and best practices for limiting Covid-19 exposure to the greatest extent possible both on complex and when the president is traveling,” he added.