Margaret Ruth Gyllenhaal (US: ; born November 16, 1977) is an American actress. Part of the Gyllenhaal family, she is the daughter of filmmakers Stephen Gyllenhaal and Naomi Achs, and the older sister of actor Jake Gyllenhaal.

She began her film career as a teenager with roles in her father's films, and appeared alongside her brother in the science fiction film Donnie Darko (2001). She garnered critical praise for starring as Lee Holloway in Secretary (2002), for which she was nominated for a Golden Globe Award. For her performance in independent drama Sherrybaby (2006), she was nominated for the Golden Globe Award for Best Actress in a Motion Picture – Drama. She also received recognition for starring as Rachel Dawes in the superhero film The Dark Knight (2008). For her performance in the musical-drama Crazy Heart (2009), she was nominated for the Academy Award for Best Supporting Actress. She subsequently starred in the fantasy comedy family film Nanny McPhee and the Big Bang (2010), the drama Won't Back Down (2012) and the comedy-drama Frank (2014), for which she was nominated for a BIFA Award.

In 2014, she made her Broadway debut in a revival of The Real Thing, and also starred in the British television BBC political spy thriller television miniseries The Honourable Woman. For her performance in the latter she won a Golden Globe Award, and was nominated for a Primetime Emmy Award.