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 and was booed by the crowd for his inactivity.
Riner dropped to his knees after the victory before hugging Mikhaylin.
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.
In the women's division, Idalys Ortiz of Cuba won the women's over 78-kilogram Olympic judo gold on Friday, improving on the bronze she won at the Beijing Games.
Ortiz defeated Japan's Mika Sugimoto in a cagey, drawn-out final with little action, where both fighters struggled to get a grip or catch the other off balance.
The match went into overtime and judges eventually ruled Ortiz the winner.
Earlier in the day, Ortiz triumphed over the top-seeded Wen Tong of China in the semifinals. Tong was the defending Olympic champion and was unbeaten at international competitions since 2007. She later won a bronze in the repechage. The other bronze medal was won by Britain's Karina Bryant.
But the victories of Riner and Ortiz were perhaps overshadowed by a first-round fight this morning that lasted little over a minute. That featured Saudi Arabia's first female judoka Olympian, Wojdan Ali Seraj Abdulrahim Shahrkhani, a blue belt who has only been training for two years.
Shahrkhani was thrown flat on her back after just 82 seconds by Puerto Rican fighter Melissa Mojica. She appeared tentative on the mat, circling Mojica and avoiding any direct attacks. Mojica finally grapped control of Shahrkhani's collar before tossing her backwards.
"I was nervous and afraid, but proud," Shahrkhani said of her fight afterwards. "I am proud to be the first Saudi woman and I'm very grateful to the crowd who supported me," she said. Shahrkhani said she intended to fight at the Rio Games next. "I will practice more," she vowed.