The crime-drama series, which is currently on its sixth season and will now return for a seventh and eighth season, stars Michael C Hall as a blood splatter expert named Dexter Morgan who also moonlights as a serial killer of criminals.
Showtime president David Nevins said: "Dexter's enormous success is a real tribute to the great achievements of its cast, producers, and the powerhouse performance of Michael C Hall.
"The series is bigger than it's ever been in its sixth season, both in terms of audience and its impact on the cultural landscape.
"Together with Michael, the creative team on the show has a very clear sense of where they intend to take the show over the next two seasons and, as a huge fan, I'm excited to watch the story of Dexter Morgan play out."
Hall added: "On behalf of the entire Dexter family, we relish the invitation to delve ever deeper into Dexter's world."
In September it was reported that Hall was close to signing a new contract with the show.
Dexter's executive producer Scott Buck revealed that they are "not working toward the end just yet".
Production for season seven will begin in 2012.
Watch a behind-the-scenes video of Dexter's season six below: