A sticky-fingered socialite is spending Christmas on Rikers Island instead of in her Southampton mansion — as punishment for swiping a pricey sweater from a Madison Avenue boutique.

Stefanie Lackner, the wife of Fisher Island Club CEO Bernard Lackner, landed 30 days in jail last week for lifting the $5,135 Brunello Cucinelli garment.

Lackner, 46, entered the high-end fashion shop a few blocks from the Douglas Elliman office where she is a Realtor in August 2012 and made the grab, according to Manhattan Criminal Court documents.

The pretty blonde first popped into a fitting room to try on the sweater, the criminal complaint said. She then requested it in another size while keeping the original sweater with her in the stall, papers state.

A police investigator who reviewed store surveillance video described the crime in court papers.

“I observed the defendant exit the fitting room with only one sweater and that the black shopping bag defendant carried into the fitting room appeared to have a bulge,” the examiner said. “The defendant did then exit the store without paying for the remaining sweater.”

Lackner wasn’t busted until that November, but it’s unclear how she was caught. Alfred Renna, Lackner’s boss at Douglas Elliman, told The Post on Tuesday that he wasn’t aware of her arrest.

“Oh, that’s a very nice store,” he said of Brunello Cucinelli, where a hooded sweat shirt retails for $3,245. “I’m certainly surprised anyone who works here was arrested.”

Lackner was originally slapped with a felony grand-larceny rap. But she was able to plead down to misdemeanor petit larceny and sentenced to a month behind bars at the Rose M. Singer Center on Rikers Island, where she’s bunking in a room with 49 other beds.

The digs are a far cry from her $1.2 million home on Long Island. And the carb-loaded food she’ll be eating behind bars also is likely to pale: Her Christmas dinner will consist of industrial-grade turkey, rice, stuffing and cranberry sauce, along with carrot cake for dessert, said a woman answering the phone there Tuesday.

Lackner and her well-heeled hotelier husband, Bernard, 59, have one child and are estranged, according to a business associate.

Lackner’s lawyer, Edward Kratt, declined to comment. Bernard Lackner did not respond to requests. A manager at Brunello Cucinelli also refused to talk.

Additional reporting by Natasha Velez