"Anthony Scaramucci is a highly unstable 'nut job' who was with other candidates in the primary who got shellaced (sic), and then unfortunately wheedled his way into my campaign," Mr Trump tweeted on Monday morning.
"I barely knew him until his 11 days of gross incompetence-made a fool of himself, bad on TV. Abused staff, got fired," the US president added without providing evidence.
"Wrote a very nice book about me just recently. Now the book is a lie? Said his wife was driving him crazy, 'something big' was happening with her. Getting divorced. He was a mental wreck. We didn’t want him around. Now Fake News puts him on like he was my buddy!"
Mr Trump's rambling attack came amid an escalating war of words between the two, which burst in to life earlier this month when Mr Scaramucci criticised the president's racist rhetoric and visits to the scenes of mass shootings.
Mr Scaramucci, who infamously kept his White House job for just 11 days, went so far as to withdraw his support for Mr Trump's re-election bid and on Monday said he was putting together a coalition of former Trump cabinet officials to denounce him ahead of 2020.
The president's mention of Mr Scaramucci's marriage referred to a brief period in 2017, when the businessman's second wife filed for divorce, before dropping the case four months later.
Deidre Ball and Mr Scaramucci eventually reconciled and the following year launched a podcast exploring their opposing political views, titled, Mooch and the Mrs.
Mr Trump followed up his tweets with another about his popularity within the Republican Party.
"Great cohesion inside the Republican Party, the best I have ever seen. Despite all of the Fake News, my Poll Numbers are great," he said.
"New internal polls show them to be the strongest we’ve had so far! Think what they’d be if I got fair media coverage!"
In fact, according to Gallup, Mr Trump's approval rating among Republicans remains at 88 per cent, which is slightly down on his high of 91 per cent, which was achieved in April