The president's long-time personal aide John McEntee has been fired by the White House, according to US media.
Mr McEntee, who served as an aide since early in the campaign, was reportedly escorted by security and was not permitted to collect his belongings.
President Donald Trump's re-election campaign announced on Tuesday Mr McEntee would join as a "senior adviser for campaign operations".
Mr McEntee was fired amid a US Department of Homeland Security investigation against him for financial crimes, CBS News reports.
US media reported his abrupt exit came about due to difficulties in obtaining a full security clearance.
The former Fox News producer joined Mr Trump's presidential campaign in the early months and had since been seen by his side at many events.
The Wall Street Journal first reported Mr McEntee's sacking, citing a security issue.
The other firing you didn't hear about
Analysis by Anthony Zurcher, BBC News, Washington
While everyone in Washington was talking about Rex Tillerson, a long-time aide to Donald Trump was shown the White House door.
John McEntee was one of the early Trump campaign true believers who followed the long-shot candidate to the White House - along with Hope Hicks and Dan Scavino. Now, only social media director Mr Scavino remains.
Details of Mr McEntee's dismissal have been scarce, with only a foreboding report that he was under investigation for "serious financial crimes" - ones not tied to any current Trump controversies.
At the very least, it appears Mr McEntee did not have a permanent security clearance and, with the ongoing criminal investigation, was unlikely ever to be granted one.
Mr McEntee's firing - which reportedly concluded with his being escorted from White House grounds - will likely renew scrutiny of the administration's background check process that first began after the termination of Rob Porter, another close Trump aide, because of his own clearance issues.
Mr McEntee won't be fully out of the Trump orbit, however. Shortly after his announced firing, he was hired by the Trump campaign - which is becoming a landing place for many of the former Trump hands who are unwilling or unable to work in the White House.
Mr McEntee, who was most recently seen travelling to Pennsylvania with Mr Trump for a rally on Saturday, was expected to join the president in California on Tuesday before his sacking.
His exit follows another original Trump campaign aide. White House Communications Director Hope Hicks resigned last month.
Mr Trump's re-election campaign manager, Brad Parscale, announced on Tuesday Mr McEntee would rejoin the 2020 effort along with Katrina Pierson, a former Trump campaign spokeswoman.
"As we build out our operations for the 2018 midterm elections and the 2020 re-election campaign, we are pleased to welcome back two outstanding members of our 2016 team," Mr Parscale said on Tuesday.
"We need the help of proven leaders such as... John to promote the President's growing portfolio of achievements across the country."