Sinéad Moira Cusack (born February 18, 1948) is an Irish actress. Her first acting roles were at the Abbey Theatre in Dublin, before moving to London in 1969 to join the Royal Shakespeare Company. She has won the Critics' Circle and Evening Standard Awards for her performance in Sebastian Barry's Our Lady of Sligo.

Cusack has received two Tony Award nominations: once for Best Leading Actress in Much Ado About Nothing (1985), and again for Best Featured Actress in Rock 'n' Roll (2008). She has also received five Olivier Award nominations for As You Like (1981), The Maid's Tragedy (also 1981), The Taming of the Shrew (1983), Our Lady of Sligo (1998) and Rock 'n' Roll (2007). In 2020, she was listed at number 25 on The Irish Times' list of Ireland's greatest film actors.