Charles Peckham Day (born February 9, 1976) is an American actor, writer, and producer. He is best known for playing Charlie Kelly on the FX/FXX dark comedy It's Always Sunny in Philadelphia (2005–present), of which he is also a writer and an executive producer. In 2011, he was nominated for a Critics' Choice Television Award and a Satellite Award for the role. Subsequently, he co-created the Apple TV+ comedy Mythic Quest (2020–2025) with Rob Mac and Megan Ganz.

In film, Day is best known for his performances as Dr. Newton Geiszler in Guillermo del Toro's Pacific Rim (2013), Dale Arbus in the comedy Horrible Bosses (2011) and its sequel Horrible Bosses 2 (2014), and Andy Campbell in the comedy Fist Fight (2017). He is also known for his voice roles in Monsters University (2013), The Lego Movie film franchise (2014–2019) and Nintendo franchise character Luigi in The Super Mario Bros. Movie (2023) and its sequel, The Super Mario Galaxy Movie (2026). He made his directorial debut with Fool's Paradise (2023).