Riner, 23, defeated Russia's Alexander Mikhaylin in an anti-climactic final where he was mainly on the attack. Mikhaylin made little effort to fight except to bat away Riner's attempts to grip his uniform.
With a record five world championship titles, Riner is judo's biggest star. He won a bronze at Beijing. On Friday, Riner used both throwing and grappling techniques to win his early fights with several match-ending ippon scores.
The bronze medals were won by Germany's Andreas Toelzer and Brazil's Rafael Silva.