Mario Casas Sierra (born 12 June 1986) is a Spanish film and television actor. Known for his roles in Spanish cinema and television, he has received various accolades, including a Goya Award, two Gaudí Awards, and three Feroz Awards.

In 2006, after featuring in minor credits in Motivos personales and Obsesión, Casas landed his first television main role in the teen drama SMS: Sin Miedo a Soñar and also debuted on the big screen in Summer Rain. Casas acquired a larger clout owing to his appearance in the television series Los hombres de Paco, as part of a popular love triangle together with Hugo Silva and Michelle Jenner. His early film career, featuring in box-office darlings such as Sex, Party and Lies (2009), Brain Drain (2009), and Three Steps Above Heaven (2010), consolidated after his performance as bad boy Hache in the latter title, which earned him wide popularity in Spain. He also starred in the television series El barco (2011−13).