Hayley Elizabeth Atwell (born 5 April 1982) is a British and American actress. After appearing in various West End productions, Atwell gained popularity for her roles in period dramas, appearing in the films Brideshead Revisited (2008), The Duchess (2008) and the miniseries The Pillars of the Earth (2010); for the latter two, she was nominated for a British Independent Film Award and a Golden Globe Award respectively.

She rose to prominence with her portrayal of Agent Peggy Carter in several Marvel Cinematic Universe (MCU) films, starting with Captain America: The First Avenger (2011), which inspired the creation of the spin-off ABC television series Agent Carter (2015–2016). Atwell also starred in the fantasy films Cinderella (2015), Christopher Robin (2018) and Peter Rabbit 2: The Runaway (2021) and had a leading role in the action film Mission: Impossible – Dead Reckoning Part One (2023). Atwell has received Laurence Olivier Award nominations for her work on stage for her leading performances in The Pride (2013) and Rosmersholm (2020).