An FDNY psychologist who says Donald Trump once crudely insulted her looks has diagnosed him as mentally unfit to be president.

Suzanne Lachmann claims her brief encounter with Trump — and her work treating FDNY firefighters after 9/11 — qualifies her to make the pronouncement.

Lachmann recently tweeted that she “met Trump personally,” and that her work with the Bravest to “determine who must be taken off line due to mental duress,” justifies her conclusion that the commander-in-chief must go.

Lachmann told The Post she spotted the famous developer standing alone at a Missoni fashion event — filled with models — on the heels of his 1992 divorce from first-wife Ivana.

She approached The Donald to introduce herself as “a neighbor,” and tell him she enjoyed his recent interview on the Howard Stern show.

“He interrupted me and said, ‘Why would I want to talk to you? Look at all the beautiful women in here. I wouldn’t let you s–k my d–k.’”

Stunned, she said, she walked away without a response.

Lachmann described the incident in a March 2017 article posted by Psychology Today.

Lachmann, 50, said she also bases her professional opinion on Trump’s many media interviews and public statements.

In February, Lachmann tweeted a rant against “Trumpculters” who “can’t distinguish what an abusive sick nut job he is.”

In an apparent effort to prove her bonafides, she attached a photo of a plaque she received from a Manhattan firehouse for her “generous time and support” after 9/11.

Lachmann said she’s paid hourly by the FDNY’s counseling services unit to treat firefighters, and is currently seeing six.

She also tweeted a photo of a parking placard issued by the United Fire Officers Association, boasting she received it “in recognition of the many FFs I’ve aided.”

She told The Post she got the placard from her ex-husband, a retired FDNY battalion chief.

Lachmann may have violated the FDNY’s social media policy, which states: “Employees who identify themselves as FDNY employees, or hold positions with the FDNY that are known to the general public … should make a clear disclaimer that the statements and views expressed … do not reflect the views of the FDNY.”

Lachmann insisted the policy does not apply to her. “I’m a consultant, not an employee,” she said.

But FDNY spokesman Frank Gribbon said, “It’s inappropriate to cite her work with FDNY in social posts that express political views.”