"They are separated," a source close to the Real Housewives of New York City stars tells E! News.
We're told, however, that the split "literally happened within the last few days," despite Mario's other relationship (and months of tabloid speculation that he was cheating).
Word of the split comes just days after Ramona cohosted an event in Manhattan.
"She hosted the Ramona Singer Wine Dinner at David Burke at Bloomingdales the night of Jan. 23," a guest at the dinner tells E! News. "But she left early, before dessert was served. She left alone."
It's believed Ramona left to head out to the Hamptons.
Meanwhile, a source tells us that Mario has been dating 27-year-old Kasey Dexter for about a year after meeting through mutual friends.
"Ramona had no idea about Kasey until the [New York Post] ran the story about Mario getting another woman pregnant over the summer," the insider said, adding that there was a pregnancy but the choice was made to end it.
The Singers have not taken any steps toward divorce yet at this time.
Meanwhile, Ramona is back for a sixth season of The Real Housewives of New York City, premiering March 11 on Bravo. She remains the show's only star to be a lead Housewife throughout the entire run.
—Reporting by Beth Sobol
(E! Online and Bravo are both members of the NBCUniversal family.)