Sierra McCormick (born October 28, 1997) is an American actress. She first became known for participating in the game show Are You Smarter Than a 5th Grader? (2007–2008) before making her acting debut with a recurring role as Lilith on the television series Supernatural (2008). She also starred as Scout Thomas on the comedy television series Romantically Challenged (2010), played Alice in the direct-to-DVD film Spooky Buddies (2011) for which she won a Young Artist Award, and Susan Kushner in the comedy film Ramona and Beezus (2010). She had her breakthrough starring as Olive Doyle on the Disney Channel series A.N.T. Farm (2011–2014).

McCormick transitioned to more mature roles by starring as Moira in the horror indie film Some Kind of Hate (2015). She then earned critical acclaim for playing Fay Crocker in the science fiction film The Vast of Night (2019) and a nomination from the Critics' Choice Super Awards. She has since played Melissa in the horror film We Need to Do Something (2021), Scarlett Winslow in three episodes of the Hulu series American Horror Stories (2021), and Sybil in the thriller film The Last Stop in Yuma County (2023). Her latest role is Mary Sue in the straight-to-video action comedy Killing Mary Sue (2025).