Morell, a 33-year veteran of the CIA who led the agency from 2010-2013, said that while in the past he has "always been silent about my preference for president," the high stakes of the 2016 election mean he can "no longer" withhold his opinion.
"On Nov. 8, I will vote for Hillary Clinton. Between now and then, I will do everything I can to ensure she is elected as our 45th president," Morell wrote.
The former CIA chief said that "Donald J. Trump is not only unqualified for the job, but he may well pose a threat to our national security."
"My training as an intelligence officer taught me to call it as I see it. This is what I did for the C.I.A. This is what I am doing now. Our nation will be much safer with Hillary Clinton as president."
He wrote, "In sharp contrast to Mrs. Clinton, Mr. Trump has no experience on national security. Even more important, the character traits he has exhibited during the primary season suggest he would be a poor, even dangerous, commander in chief."
Morell also suggested that Trump is being played by Russian President Vladimir Putin, writing: "President Vladimir V. Putin of Russia was a career intelligence officer, trained to identify vulnerabilities in an individual and to exploit them. That is exactly what he did early in the primaries. Mr. Putin played upon Mr. Trump's vulnerabilities by complimenting him. He responded just as Mr. Putin had calculated."
"In the intelligence business, we would say that Mr. Putin had recruited Mr. Trump as an unwitting agent of the Russian Federation," Morell said.
Morell hailed Clinton for being "prepared, detail-oriented, thoughtful, inquisitive, and willing to change her mind if presented with a compelling argument," citing his time working with her when she was secretary of state. "I never saw her bring politics into the Situation Room," he wrote.
Trump's running mate Mike Pence, the Indiana governor, criticized Morell in an interview on NBC's "Today" Friday.
"Honestly, the comment by the former CIA official, I suppose this is the same CIA that told the president that ISIS was the JV team," Pence said.
He disputed Morell's characterization of Trump as unwittingly doing Russia's bidding.
"People that know Donald Trump know that he knows how to stand up and he knows how to stand strong, and standing up to Russian aggression is going to be really different under a Trump-Pence administration, and everybody knows that," Pence said.