Hillary Clinton says Donald Trump lacks the “temperament” to be president, but a Secret Service veteran posted outside Bill Clinton’s Oval Office in the ’90s is making that charge against . . . her.

And it rings all too true.

In his book due out June 28, “Crisis of Character,” former Secret Service Officer Gary Byrne says Hillary’s just too “erratic, uncontrollable and occasionally violent” to be trusted with the White House. In Monday’s Post, Emily Smith, citing excerpts obtained by Page Six, reported Byrne’s allegations. They’re not pretty.

Hillary “lacks the integrity and temperament to serve,” Byrne writes. “From the bottom of my soul I know this to be true.”

Her leadership style, he says, is “volcanic, impulsive, enabled by sycophants, and disdainful of the rules set for everyone else.” She shifted from friendly to raging on a dime. She screamed obscenities at Bill, Secret Service agents and White House staffers.

He tells of one “violent encounter” between the couple and notes that Secret Service personnel feared they might have to protect Bill from her attacks.

Yet last week, Hillary claimed Trump is too “dangerous,” thin-skinned and “temperamentally unfit” to serve and be given the nuclear codes.

Byrne’s not alone. Ronald Kessler’s book, “First Family Detail,” cites similar descriptions by Secret Service agents. Kate Andersen Brower, in “The Residence,” relates a story of a vicious fight between the Clintons behind a closed door. Ed Klein’s book about Hillary is simply titled: “Unlikeable.”

Byrne’s a 29-year veteran of the military and law enforcement. But what gives his claims extra credibility is Hillary Clinton’s own history — from her disdain for the rules (see: Emailgate) to her public smears of inconvenient people (see: Monica Lewinsky) to her indifference and lies about key issues (see: Benghazi).

So Byrne is just corroborating what so many already know: Hillary can’t be trusted.