Beijing will be the venue for the 2008 Olympic Games, and currently has a population of 15m.
Future Viacom Outdoor boss Leslie Moonves said: "This purchase is now a significant foothold for us in a nation where the growth opportunities are virtually limitless."
When Viacom splits into two companies, Viacom Outdoor will become a unit within the new CBS Corporation, of which Moonves will be CEO.
The Magic Media deal is expected to be completed in October, and contains an option for Viacom to acquire the remaining 30% in five years' time.
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