Walters is retiring that day after a storied television reporting career spanning five decades.
De Blasio made the announcement Monday when he appeared on "The View," the daytime talk show that Walters co-hosts, presenting her with an official city proclamation.
The 84-year-old TV veteran and de Blasio's predecessor, Michael Bloomberg, had a frequently flirtatious on-air relationship.
De Blasio and his wife, Chirlane McCray, appeared together on the long-running ABC talk show. The show's hosts mostly wanted to ask about the couple's relationship and family, and the conversation largely avoided politics. De Blasio, who is white, and McCray, who is black, spoke about the challenges of being an interracial couple.
On May 15, all 11 past and present hosts of "The View" will appear together for the first time ever to salute Walters.