The company, which owns CBS and MTV Networks, has said that Tom Freston and Leslie Moonves have been appointed as co-presidents and co-chief operating officers. Freston is currently chairman and chief executive officer of MTV Networks, while Moonves holds the same position at CBS.
Karmazin's resignation comes after wide reports of tension between him and Redstone. In a statement, Karmazin, who has been with Viacom for more than 20 years, said: "For personal and professional reasons, I have decided to leave Viacom and pursue other challenges."
The appointments of Moonves and Freston are part of a plan to structure an orderly succession, with Redstone, a stalwart of the company, planning to step down within the next three years.
Redstone said that he regretted Karmazin's decision to step down, but said: "I am very excited about working with Tom, Les and the other great executives at Viacom to bring Viacom to a new level of performance based on the creative excellence and innovation that has always been at the core of my vision for this company."
Moonves is familiar to US television audiences after becoming the subject of a series of jokes by late night television host David Letterman.
He joined CBS in July 1995 as president of CBS Entertainment and took up his current role in April last year. Freston has held a succession of positions with MTV Networks, before being appointed to the chief executive role in 1987.
Karmazin is to stay with the company for two months and assist Moonves and Freston in the transition to their new roles.
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