McKinnon, 28, would be the first out lesbian in the show's 37-year history. Before UCB, McKinnon, a Columbia University grad, had various acting and improv gigs, including Logo's "The Big Gay Sketch Show."
Deadline.com -- which first reported the hire Thursday -- said McKinnon is expected to join for the April 7 show. That edition will be hosted by "Modern Family's" Sofia Vergara, with British boy band One Direction as the musical guest.
A "Saturday Night Live" spokeswoman declined to comment. McKinnon -- whose given name is Kate McKinnon Berthold -- also declined to comment when reached by phone late Thursday.
The contract of Kristen Wiig, "Saturday Night Live's" most prominent female performer, expires at the end of this season.