White House Chief of Staff Mark Meadows has been diagnosed with coronavirus.
Mr Meadows was at a White House party on election night and was seen without a mask.
He has frequently appeared at events without a face covering and accompanied the president in the final days of the campaign rallies.
Mr Meadows last appeared in public on Wednesday morning, again without a mask, as the president attempted to claim victory in the election.
The diagnosis was confirmed on Friday by two senior White House officials.
Another top campaign official, Nick Trainer, has also tested positive, a source told the Reuters news agency.
Mr Meadows’ condition and the circumstances of how he might have become infected are so far unclear.
He is the latest of Mr Trump’s inner circle to test positive.
Before the latest two cases were confirmed, 28 people connected to the Trump administration or their contacts had tested positive for COVID-19 in late September or early October, according to NBC News.
At least 14 of those cases had attended the event in the Rose Garden of the White House for Mr Trump’s Supreme Court nominee Amy Coney Barrett on 26 September.
They included President Donald Trump and First Lady Melania Trump, press secretary Kayleigh McEnany, Mr Trump’s adviser Chris Christie, former presidential adviser Kellyanne Conway and senators Mike Lee and Thom Tillis.
Guests at the event were seen hugging, fist-bumping and not wearing masks.
Top US infectious diseases expert, Dr Anthony Fauci, later described it as a “superspreader event”.
Others in Mr Trump’s inner circle who contracted the virus were his son Barron, national security adviser Robert O’Brien and the vice president’s chief of staff Marc Short.
There have been more than 236,000 coronavirus-linked deaths in the US so far this year, according to Johns Hopkins University, and new cases have increased significantly in the last two weeks.
The US seven-day average is now around 95,000 per day.
Joe Biden – speaking on Friday evening as he appeared on the brink of winning the election – said cases were “skyrocketing”.
“I want everyone to know that on day one, we are going to put our plan to control this virus into action,” he said.
“That can’t save any of the lives that have been lost, but it will save a lot of lives in the months ahead.”
Mr Trump has argued that travel restrictions he brought in at the start of the pandemic saved thousands of lives.
However, his early refusal to wear a mask is one of many things criticised by those opposed to his handling of the crisis.