Mercedes Alicia McNab (born March 14, 1980) is a Canadian-born actress perhaps known for playing the role of Harmony Kendall on Buffy the Vampire Slayer and its spinoff Angel. More minor television roles include a part on Boston Public where her character posed nude for an adult magazine.

Before her television work, McNab played a Girl Scout selling cookies in one of the better-known scenes from the 1991 film The Addams Family. She enjoyed a more prominent role as a stuck-up summer camp resident in its 1993 sequel, Addams Family Values. She posed nude for, and was on the cover of the November 2006 issue of Playboy magazine. McNab also appeared in the January 2004 issue of FHM magazine.

McNab was born in Vancouver, British Columbia; her father is English football player Bob McNab.

Mercedes McNab has been acting professionally since she was 10 years old. Born and raised in Vancouver, British Columbia, Canada, she moved with her parents to Los Angeles at age nine where she had a desire to become an actress and soon landed an agent. Her first role, at age 10, is probably the one where she is most widely recognized for in a bit part as the young girl scout in the hit film, Addams Family, The (1991). She even landed a similar role as Amanda in the sequel, Addams Family Values (1993). But the role she is most famous for is a recurring role in playing the dim-witted Harmony Kendall on "Buffy the Vampire Slayer" (1997) for four years, and later its spin-off "Angel" (1999) in a guest spot in 2001 and later as a regular for its final season from 2003-2004. Some of her television appearances include the unaired Fox television pilot for "Buffy the Vampire Slayer" (1997), and appearances in "Harry and the Hendersons" (1991), "Touched by an Angel" (1994) and "Walker, Texas Ranger" (1993).