Charles Peckham Day (born February 9, 1976) is an American actor, comedian, writer, producer, and podcaster. He is best known for playing Charlie Kelly on the FX comedy It's Always Sunny in Philadelphia (2005–present), which he co-created with Rob McElhenney and Glenn Howerton, and on which he is also executive producer and writer. In 2011, he was nominated for a Critics' Choice Television Award and a Satellite Award for the role. He subsequently co-created The Cool Kids (2018–2019) on Fox with Paul Fruchbom in 2018 and Mythic Quest (2020–present) on Apple TV+ with Rob McElhenney and Megan Ganz in 2020, and continues to executive-produce the latter.

On film, he is best known for his performances as biologist Dr Newton Geiszler in Guillermo del Toro's science-fiction monster movie Pacific Rim (2013) and its sequel Pacific Rim Uprising (2018), Dale Arbus in the comedy Horrible Bosses (2011) and sequel Horrible Bosses 2 (2014), and teacher 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). He made his directorial debut with Fool's Paradise in 2023.