Charles Peckham Day (born February 9, 1976) is an American actor, 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) and The Lego Movie film franchise (2014–2019). He will voice Luigi in the upcoming Nintendo film The Super Mario Bros. Movie in April 2023, and also make his directorial debut with Fool's Paradise slated for release in May 2023.