Hope Hicks, who serves as counsellor to the president and travelled with him to a rally on Wednesday, tested positive on Thursday, according to an administration official, who spoke on condition of anonymity.
The 31-year-old is the closest aide to Mr Trump to test positive so far.
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”.
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.
Ms Hicks traveled 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.
Ms Hicks, one of the president’s most trusted aides, previously served as White House communications director and rejoined the administration this year ahead of the election.
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.
