Gillian Leigh Anderson OBE (born August 9, 1968) is an American actress, writer, and activist. She is best known for her roles as FBI Special Agent Dana Scully in the sci-fi series The X-Files (1993–2002; 2016–2018), Lily Bart in the drama film The House of Mirth (2000), DSI Stella Gibson in the BBC/RTÉ crime drama series The Fall (2013–2016), Jean Milburn in the Netflix comedy-drama series Sex Education (2019–2023), and Margaret Thatcher in the fourth season of the Netflix drama series The Crown (2020). She has won two Primetime Emmy Awards, two Golden Globe Awards, and four Screen Actors Guild Awards.

Born in Chicago, Anderson was raised first in London and then in Grand Rapids, Michigan. She later started her career onstage in New York City before achieving international recognition for her work on The X-Files. Her film work includes the dramas The Mighty Celt (2005), The Last King of Scotland (2006), Shadow Dancer (2012), and Viceroy's House (2017), as well as the X-Files films Fight the Future (1998) and I Want to Believe (2008). Her television credits include Lady Dedlock in Bleak House (2005), Wallis Simpson in Any Human Heart (2010), Miss Havisham in Great Expectations (2011), Dr. Bedelia Du Maurier on Hannibal (2013–2015), Media in the first season of American Gods (2017), and Eleanor Roosevelt on The First Lady (2022).

Anderson has also received awards and acclaim for her stage work, which includes Absent Friends (1991), for which she won a Theatre World Award for Best Newcomer; A Doll's House (2009), for which she was nominated for a Laurence Olivier Award for Best Actress; Blanche DuBois in A Streetcar Named Desire (2014 and 2016), for which she won the Evening Standard Theatre Award for Best Actress and received a second Laurence Olivier Award nomination; and All About Eve (2019), for which she received a third Laurence Olivier Award nomination.

Anderson has supported numerous charities and humanitarian organizations, being an honorary spokesperson for the Neurofibromatosis Network and a co-founder of South African Youth Education for Sustainability (SAYes). She has lived in London since 2002 and was appointed an honorary Officer of the Most Excellent Order of the British Empire (OBE) in 2016 for her services to drama.