Sarah Ellen Polley (born January 8, 1979) is a Canadian actress, writer, director, and political activist. Polley first garnered attention and is probably most known as a child actress for her role as Ramona Quimby in the television series Ramona, based on Beverly Cleary's books. Subsequently this lead on to her role as Sara Stanley in the Canadian television series Road to Avonlea (1990–1996). She has starred in many feature films, including Exotica (1994), The Sweet Hereafter (1997), Guinevere (1999), Go (1999), The Weight of Water (2000), My Life Without Me (2003), Dawn of the Dead (2004), Splice (2009), and Mr. Nobody (2010).

Polley made her feature film directorial debut with Away from Her (2006), for which she won a Genie Award for Best Achievement in Direction and was nominated for an Academy Award for Best Adapted Screenplay. Polley's second film, Take This Waltz (2011), premiered at the 2011 Toronto International Film Festival.

Her first documentary film, Stories We Tell (2012), was awarded the $100,000 prize for best Canadian film of the year by the Toronto Film Critics Association. In 2017, Polley executive produced the film A Better Man (2017), and wrote the miniseries Alias Grace, based on the 1996 novel of the same name by Margaret Atwood, which Polley began adapting in 2012.