Louise Brealey (born 27 March 1979) is an English actress, writer and journalist. She played Molly Hooper in Sherlock (2010–), Cass in Back (2017), Scottish professor Jude McDermid in Clique (2017), Gillian Chamberlain in A Discovery of Witches (2018) and Donna Harman in Death in Paradise (2020).

She made her TV debut as Nurse Roxanne Bird in two series of BBC drama Casualty (2002–2004) before playing Judy Smallweed in Bleak House (2005). Brealey plays pathologist Molly Hooper in all four series of Steven Moffat and Mark Gatiss's television drama, Sherlock. She has also made her name as an accomplished stage actress.