The 60-year-old executive, who was a founder of MTV, is being replaced by Philippe P. Dauman, 52, Viacom said Tuesday in a statement.
The board also named Thomas E. Dooley, 49, to the newly created position of senior executive vice president and chief administrative officer.
Dauman will report to Sumner M. Redstone, Viacom's executive chairman and founder. Dooley will report to Dauman.
"I've spent over 26 years at Viacom, 18 of them with Sumner," Freston said. "With my exceptional colleagues, we built a worldwide powerhouse of brands and businesses, literally from scratch. I leave many good friends knowing that they have an unmatched track record, a great plan going forward and incredible abilities to execute on it in this digital age. I have worked closely with Philippe Dauman and Tom Dooley over the years and have the highest respect for their abilities."