Sarah Ellen Polley OC (born January 8, 1979) is a Canadian filmmaker, writer, political activist and actress. She first garnered attention as a child actress for her role as Ramona Quimby in the television series Ramona, based on Beverly Cleary's books. This subsequently led to her role as Sara Stanley in the Canadian television series Road to Avonlea (1990–1996). She has starred in many feature films, including The Adventures of Baron Munchausen (1988), Exotica (1994), The Sweet Hereafter (1997), Guinevere (1999), Go (1999), The Weight of Water (2000), No Such Thing (2001), My Life Without Me (2003), Dawn of the Dead (2004), Splice (2009), and Mr. Nobody (2009).

Polley made her feature film directorial debut with Away from Her (2006), for which she won the Canadian Screen Award for Best Director and was nominated for the Academy Award for Best Adapted Screenplay. Her second film, Take This Waltz (2011), premiered at the 2011 Toronto International Film Festival, followed by her first documentary film, Stories We Tell (2012). She also wrote the miniseries Alias Grace, based on the 1996 novel of the same name by Margaret Atwood. In 2022, she wrote and directed the film Women Talking, based on the 2018 novel of the same name by Miriam Toews, for which she won the Academy Award for Best Adapted Screenplay.

Sarah Ellen Polley was born on January 8, 1979, in Toronto, Ontario, Canada, the youngest of five children born to Diane Elizabeth Polley (née MacMillan of Clan MacMillan). Her siblings are Susy and John Buchan from Diane's first marriage to George Deans-Buchan, and Mark and Joanna Polley from her second marriage to Michael Polley (1933–2018), a British-born actor who became an insurance agent after starting a family with Diane.

Her mother was an actress (best known for playing Gloria Beechham in 44 episodes of the Canadian TV series Street Legal) and a casting director. She died of cancer the week of Polley's 11th birthday in 1990.

Polley suffered from severe scoliosis as a child and underwent a spinal operation at 15 that required her to spend the next year in bed recovering.

Polley was raised by Diane and Michael. During her childhood, Polley's siblings teased her because she bore no physical resemblance to Michael. Polley discovered as an adult that her biological father was actually Harry Gulkin, with whom her mother had an affair (as chronicled in Polley's film Stories We Tell). Gulkin, the son of Russian Jewish immigrants, was a Quebec-born film producer who produced the 1975 Canadian film Lies My Father Told Me, and had met Diane after attending a play in which she acted in Montreal in 1978. When Polley turned 18, she decided to follow up on suggestions from her mother's friends that her biological father might be Geoff Bowes—one of three castmates from her mother's play in Montreal. Meeting with Gulkin as just someone who could provide information about Diane in Montreal, he informed Polley of his affair with Diane. Gulkin's paternity was later confirmed by a DNA test.

Polley attended Subway Academy II, then Earl Haig Secondary School, but dropped out at age 15. By the age of 15 she was living on her own and credits the Ontario Coalition Against Poverty for housing her and developing her work with activism.

In November 2024, Polley received an honorary Doctors of Letters degree from the University of British Columbia Vancouver campus.