Karen Sheila Gillan (; born 28 November 1987) is a Scottish actress. Gillan gained recognition for her work in British film and television, particularly for playing Amy Pond, a primary companion to the Eleventh Doctor in the science fiction series Doctor Who (2010–2013), for which she received several awards and nominations. Her early film roles include Ally in the thriller film Outcast (2010) and Jane Lockhart in the romantic comedy film Not Another Happy Ending (2013). She also worked on the stage while in Britain, appearing in John Osborne's play Inadmissible Evidence (2011) before making her Broadway debut in the play Time to Act (2013).

Gillan made her transition to Hollywood starring as Kaylie Russell in the horror film Oculus (2013), her first commercial success in the United States, and thereafter played the lead in the sitcom Selfie (2014). She achieved international stardom for portraying Nebula in the Marvel Cinematic Universe superhero films Guardians of the Galaxy (2014), Guardians of the Galaxy Vol. 2 (2017), Avengers: Infinity War (2018), and Avengers: Endgame (2019), the latter of which is one of the highest-grossing films of all time; she will reprise the role in the upcoming films Thor: Love and Thunder (2022) and Guardians of the Galaxy Vol. 3 (2023). She additionally was praised for her portrayals of Ruby Roundhouse in the action films Jumanji: Welcome to the Jungle (2017) and Jumanji: The Next Level (2019), Sam in the comedy film Gunpowder Milkshake (2021) and her double role in the sci-fi thriller film Dual (2022).

Gillan's accolades include an Empire Award, a National Television Award, a Teen Choice Award and nominations for a British Academy Scotland Film Award, a Critics' Choice Award and a Saturn Award. Aside from acting, she garnered critical acclaim for her involvement as writer and director in the drama film The Party's Just Beginning (2018), which she also headlined. She has been noted for her public image and activism, particularly towards suicide prevention.