Comey made the pronouncement before a packed house at a Baltimore Speakers Series talk at Meyerhoff Symphony Hall.
“I don’t buy the stuff about mental incompetence,” he said during a question-and-answer period.
“I don’t think he’s mentally incompetent. I think he’s morally incompetent to be president.”
Comey — fired by Trump last year — said he reached his conclusions because of Trump’s “values.”
He said the president lies constantly, treats women like “meat” and failed to properly condemn white supremacists who rallied in 2017 in Charlottesville, Va., where a counterprotester was killed.
But Comey also said Trump shows his intelligence because he “communicates with people in a direct way,” an apparent reference to the president’s frequent tweets and campaign-style rallies.
Appointed FBI director in 2013, Comey was fired in May 2017.
At the time, Comey was leading a criminal investigation into possible collusion between Russians and Trump associates during the 2016 presidential campaign.
Trump criticized Comey over his handling of an investigation into a private email server used by Hillary Clinton, the 2016 Democratic presidential nominee. But Comey later told a Senate committee, "It's my judgment that I was fired because of the Russia investigation.”
Wearing a dark suit, the 6-foot-8 Comey stood onstage and spoke about his experiences with Trump and other presidents. He later took written questions from the audience.
Most in the crowd stood and applauded when he was done.
Comey said his relationship with Trump was different than with Obama and Bush.
His encounters with Trump, he said, amounted to “him trying to pull me in close. I kept trying to push away.”
Comey said his distance was in keeping with the independence traditionally afforded the FBI director.
Obama and Bush “were scrupulous about maintaining that distance,” he said.
Comey said he learned something else about Trump. He was, in effect, a one-way transmitter.
“To tell the man almost anything you had to interrupt him. To my mind, that’s a manifestation of insecurity,” Comey said.
Obama, by contrast, was a great listener and asked lots of questions, he said.
Comey also waded into the Supreme Court nomination of Brett Kavanaugh.
The FBI is currently conducting a review into sexual assault allegations against the nominee before the Senate decides whether to confirm him.
The review is unusual because it is limited to less than one week.
But Comey said he believes the FBI can produce something useful in that time.
“It can be done if they’re allowed to do their job,” he said. “They’ll learn more from voluntary interviews than you might expect.”