The Real Housewives of New York City star ended her relationship with Irwin late last year, Us Weekly reports.
In a statement, Radziwill confirmed: "Russ and I have decided to call it quits."
The couple had been dating for about two years, but Radziwill joked that she isn't worried about Valentine's Day on Thursday.
"I haven't celebrated Valentine's Day since the fifth grade when the boy I had a crush on gave me a valentine, only to find out later that he gave one to the entire class, including the boys," she quipped.
Radziwill is currently working on her novel The Widow's Guide to Sex and Dating, which is due to be released in July.