The world number one romped home in a new personal best time of 12.92 seconds, with USA team-mate Jason Richardson adding an Olympic silver to his world title with a time of 13.04.
Jamaica's Hansle Parchment took bronze in a national record time of 13.12.
"The gold medal means everything," Merritt said. "The U.S. haven't had a gold medal since Allen Johnson in 1996. It's phenomenal. To be here in this atmosphere is really special."
Young Briton Lawrence Clarke - a surprise qualifier after setting a personal best in the semis - came a creditable fourth although some way off the medals.
Cuba's Robles, who win the 2008 title in Beijing and is the world record holder, pulled up injured after tripping on the sixth hurdle.
He was disqualified at the World Championships in Daegu and, having long-standing injury problems, may walk away from the sport after losing both his major titles.
Former world record holder and 2004 Olympic champion Liu Xiang failed to get out of the first heats with an Achilles injury, while Britain's former European champion Andy Turner missed out after failing to qualify from his semi final.
But Merritt, most experts' tip for gold this year, lived up to the hype as the USA return to the top of sprint hurdling after losing their way after Allen Johnson's retirement and David Oliver's constant failures in major championships.