Comey, who is promoting a new tell-all book, was speaking in his first major TV interview since being fired by Trump last May.
Former FBI Director James Comey has slammed Donald Trump as "morally unfit" to be US President.
He was speaking in an exclusive interview with ABC News' George Stephanopoulos, broadcast on Sunday.
"I don't think he is medically unfit to be president. I think he is morally unfit to be president," the ex-FBI chief declared as he promoted his new tell-all book 'A Higher Loyalty,' which is due out on Tuesday.
Fired by Trump last May, Comey was worried the president may be open to blackmail by Russia given claims the then-businessman was present when prostitutes urinated on each other during a 2013 Moscow visit.
Comey said that it is "possible, but I don't know" whether Russia had evidence to back up the allegations.
Trump told Comey that he had not stayed overnight in the Moscow hotel and that the claims were not true, Comey said.
"A person ... who talks about and treats women like they're pieces of meat, who lies constantly about matters big and small and insists the American people believe it, that person's not fit to be president of the United States, on moral grounds," Comey said.
"Our president must embody respect and adhere to the values that are at the core of this country, the most important being truth."
Comey claims Trump, in a private meeting, had pressed him for his personal loyalty.
"He asked for my loyalty personally as the FBI director. My loyalty's supposed to be to the American people and to the institution," Comey said in the ABC interview.
The FBI has long tried to operate as an independent law enforcement agency.
Ahead of the broadcast, Trump took to Twitter to deny he had asked for Comey's personal loyalty and brand him a liar and 'slimeball', suggesting he deserved jail and was the worst FBI Director in history.
Comey's firing came as the FBI was probing possible connections between Trump's 2016 presidential campaign and Russia's alleged meddling in the US election.
It led to the appointment of Special Counsel Robert Mueller to investigate the claims.
Russia has denied interfering in the election and Trump has denied any collusion or improper activity.