Katherine Evelyn Anita Cassidy (born November 25, 1986) is an American actress. After initially appearing in minor television roles, she made her film debut in When a Stranger Calls (2006). During the same year, she also had her first starring role as Kelli Presley in Black Christmas (2006). In 2007, Cassidy gained attention for her role as Ruby on the supernatural-horror television series Supernatural, during its third season. She later had a supporting role in Taken. In 2009, Cassidy became a cast member of the series Harper's Island and Melrose Place, both of which lasted for only one season. During 2010, she had a supporting role as Kris Fowles in A Nightmare on Elm Street and a recurring role as Juliet Sharp during the fourth season of the CW teen drama Gossip Girl.

Cassidy had her breakthrough in 2012, after being cast as Laurel Lance / Black Canary, one of the main protagonists on the television series Arrow, which spawned an entire franchise known as the Arrowverse. Cassidy reprises her role in The Flash, Legends of Tomorrow and voiced the character in the animated web series Vixen. Following her character's death, Cassidy began playing Laurel Lance / Black Siren, an antagonistic version of her character from a parallel universe, starting in The Flash and continuing on to Arrow.