Vivendi SA is working with two investment banks to explore the sale of videogame giant Activision Blizzard Inc., in which the French conglomerate owns a majority stake, people familiar with the matter said.

Goldman Sachs Group Inc. and Barclays PLC are helping Paris-based Vivendi assess its options around Activision Blizzard, which has a market capitalization of about $13.4 billion. Vivendi owns a 61% stake through a 2008 merger of its gaming unit, Blizzard Entertainment, with Activision.

Vivendi...