World youth champion Rissveds, 22, finished 37 seconds ahead of Poland's Maja Wloszczowska, a silver medallist also in Beijing in 2008, and close to a minute and a half in front of Catherine Pendrel of Canada.
The win was a first gold medal for Sweden in any Olympic cycling event since Bernt Johansson who won the men's road race in 1976 in Montreal.
“I just came here a week ago. I crashed in training, and ended up with six stitches in my knee and four in my elbow and I thought this is not going to work at all,” said the new champion.
“But the day after that I went out on the course and I felt so good. I like this course but I was a little bit scared after that. After a few laps I felt good enough.”
In an indication of the bruising nature of the sport, Wloszczowska missed the London Games with a leg fracture.
“Before London I was one of the favourites for the gold medal, but I broke my leg three weeks before the Games, so I had to focus for another four years,” said the Pole.
“It was not easy but it was all needed to be lucky today.”