HE’S called her Gustav Klimt’s Mona Lisa – and as cosmetics king and Neue Galerie President Ronald Lauder yesterday unveiled the painting for which he reportedly paid a record $135 million, Adele Bloch-Bauer did seem to have a certain slight, knowing smile.

This was the woman, after all, whom the painter may have slept with – the beautiful, 17-years-younger wife of the Jewish sugar magnate who’d commissioned the painting, and the only woman Klimt painted twice in full-length portraits. (The artist never married, but it didn’t stop him from fathering 14 children.)

The portrait took Klimt three years to finish. After Adele died of meningitis in 1925, at age 43, it hung in the Bloch-Bauers’ bedroom, only to be seized later by the Nazis.

For more than 50 years, “Adele Bloch-Bauer 1” hung in Austria’s National Gallery, until a prolonged legal battle led to the ruling this year that turned it over to Adele’s heirs, who in turn took it to Christie’s. The widely reported auction price of $135 million is the most ever paid for a single painting.

“As you can see, it works very well in this room,” Lauder proclaimed yesterday at a press preview – and it did, surrounded as it was by marble walls and several other Klimt portraits.

Amid the mob of reporters and photographers who clustered around Lauder and the portrait was a delegation from Austrian radio and TV.

“I think most people in Austria are sad but understand that this is justice,” Raimund Loew, bureau chief of ORF Radio Austria, told The Post before rushing to interview the painting’s new owner.

Also present was Adele’s only surviving heir, her 90-year-old niece, Maria Altmann – looking terrific in a light olive suit – who offered a few words about the woman she knew as Auntie.

“I was too young to get to know her, but my older sister did,” she observed of the porcelain-skinned, dark-eyed Adele, who died when Marie was 9.

“She surrounded herself with people of science and art . . . I remember she always carried a long cigarette holder.”

The portrait was displayed in Los Angeles for a time before coming to New York.

Now it’s here to stay, in the elegantly appointed museum at Fifth Avenue and 86th Street.

“I’m so happy that [these paintings] are here in New York,” Altmann says, “where everyone can see them.”