Viacom Inc. (NASDAQ: VIA, VIAB), short for "Video & Audio Communications", is an American global mass media company with interests primarily in, but not limited to, cinema and cable television. As of 2010, it is the world's fourth-largest media conglomerate, behind The Walt Disney Company, Time Warner and News Corporation. Viacom is owned in majority by National Amusements, Inc., a privately owned theater company based in Dedham, Massachusetts, USA. National Amusement holds another controlling stake in CBS Corporation.

The current Viacom was created on December 31, 2005 as a spinoff from CBS Corporation, which changed its name from Viacom to CBS at the same time. CBS, not Viacom, retains control of the over-the-air broadcasting, TV production, outdoor advertising, subscription pay television (Showtime) and publishing assets (Simon & Schuster) formerly owned by the larger company. However, Sumner Redstone through National Amusements, retains majority control of Viacom. Predecessor firms of Viacom include Gulf+Western, which later became Paramount Communications Inc., and Westinghouse Electric Corporation.