Sienna Rosie Diana Miller (born December 28, 1981) is an American-born English actress. Born in New York City and raised in London, she began her career as a photography model, appearing in the pages of Italian Vogue and for the 2003 Pirelli calendar. Her acting breakthrough came in the 2004 films Layer Cake and Alfie. She subsequently portrayed socialite Edie Sedgwick in Factory Girl (2006) and author Caitlin Macnamara in The Edge of Love (2008), and was nominated for the 2008 BAFTA Rising Star Award. Her role as The Baroness in G.I. Joe: The Rise of Cobra (2009) was followed by a brief sabbatical from the screen amid increased tabloid scrutiny.

Miller returned to prominence with her role as actress Tippi Hedren in the television film The Girl (2012), for which she was nominated for the BAFTA Television Award for Best Actress and the Golden Globe Award for Best Actress – Miniseries or Television Film. Further critical acclaim followed throughout the 2010s, with appearances in the films Foxcatcher (2014), American Sniper (2014), Mississippi Grind (2015), The Lost City of Z (2016), Live by Night (2016), and American Woman (2018), as well as the miniseries The Loudest Voice (2019).