"Their business model, which is based on building traffic and selling advertising off of unlicensed content, is clearly illegal and is in obvious conflict with copyright laws," the media-entertainment firm said in a statement.
Viacom filed suit in a New York district court accusing YouTube and Google of "massive intentional copyright infringement" of its entertainment properties.
The suit seeks more than one billion dollars in damages, as well as an injunction prohibiting Google and YouTube from further copyright infringement.
Last month, YouTube said it would remove more than 100,000 unauthorized clips of Viacom television programs after a promised copyright protection system was not installed on the popular website.
YouTube has stepped up its efforts to remove copyrighted material, and to tie up with entertainment providers, since it was taken over by Google for 1.65 billion dollars in November.
The website says it has concluded more than 1,000 partnerships with content providers, both big and small, including heavyweight broadcasters the BBC, NBC, CBS and Fox.