Luann de Lesseps’ ex-husband is finally admitting that he cheated on her — but only because, he claims, she cheated on him first.

Page Six is told that Tom D’Agostino Jr. is furious that de Lesseps claimed in her “Countess and Friends” cabaret show that he was unfaithful. He’s been telling pals that she stepped out on him first, and he only strayed because she did.

Perhaps that kind of maturity explains why their marriage only lasted eight months. They married on New Year’s Eve 2017 and split in August of that year.

Meanwhile, we’re told that D’Agostino has also been nastily mocking de Lesseps, who’s been in and out of rehab since the end of their marriage. He’s “joked” to pals, “I didn’t realize I was marrying a Walgreens!,” claiming that she had more pills around the house than the pharmacy chain.

De Lesseps has said she struggles with alcohol problems, but there has never been any suggestion that she has an issue with drugs or pills.

Reached for comment, de Lesseps told Page Six, “I wish [D’Agostino] well. I have moved on, and I hope he can do the same.”