The president tweeted praise for his current secretary of state, Mike Pompeo, while directing a torrent of abuse toward his predecessor.
“Rex Tillerson, didn’t have the mental capacity needed,” Mr Trump said.
“He was dumb as a rock and I couldn’t get rid of him fast enough. He was lazy as hell. Now it is a whole new ballgame, great spirit at State!”
The outburst came less than a day after Mr Tillerson shared some unflattering observations about his former boss with CBS host Bob Schieffer.
He told described Mr Trump as a “challenging individual” who is “undisciplined, doesn’t like to read, doesn’t read briefing reports”.
Mr Tillerson added that the president “doesn’t like to get into the details of a lot of things”, instead acting on what he believes.
As secretary of state, Mr Tillerson said he found himself warning the president that many of his proposed plans were likely illegal.
“The president will say here’s what I want to do and here’s how i’m going to do it,” Mr Tillerson said.
“I’d have to say to him, ‘Well, Mr. President, I understand what you want to do, but you can’t do it that way it violates the law, it violates a treaty’.
“He got really frustrated.”
Mr Trump fired Mr Tillerson from his role in March, after a series of public disagreements over North Korea, Russia and Iran.
It came amid reports that Mr Tillerson had called Mr Trump a “moron”.
Mr Tillerson claimed in the CBS interview that he had never met Mr Trump before he was appointed by the president in February 2017.