(Recasts with details of vote)
May 17 (Reuters) - CBS Corp's board on Thursday approved a special dividend that would cut controlling shareholder Shari Redstone's stake in the U.S. media company to about 20 percent, a move that requires approval by a Delaware court.
The vote was an act of defiance aimed at preventing a merger with sister company, Viacom Inc.
The Redstone family's National Amusements Inc controls both CBS and Viacom. NAI has said it wants to merge them if both companies support the deal.
"The board of directors has taken this step because it believes it is in the best interests of all CBS stockholders, is necessary to protect stockholders interests and would unlock significant stockholder value," CBS said in a statement.
(Reporting by Tom Hals in Wilmington, Delaware, Jessica Toonkel in New York and Lisa Richwine in Los Angeles; editing by Tom Brown and Lisa Shumaker)