Martynov became the second shooter in Olympic history to score the maximum 600 points in qualification, and went on to score a total of 705.5 points, beating the 12-year-old best mark from Germany's Christian Klees by 0.7.
"Obviously I am proud of the record and the medal," said Martynov, who looked as calm and composed while carrying his gold medal as he had been during the competition shortly before.
"This is the type of sport where you have to be calm, also after winning," he said through an interpreter.
Martynov has now shot the perfect 600 six times — more than any other shooter. He shot his first at a World Cup event in Munich, Germany in 1997.
The 44-year-old Martynov, who first competed in the Olympics in the 10M air rifle in Seoul in 1988, has been ever present in the 50M rifle prone since the 1996 Atlanta Games. He won bronze in 2000 and 2004, but finished eighth in Beijing four years ago.
As the top-ranked shooter, Martynov was regarded a favourite for the title.
"There might be something in that," he said. "But I did not expect to win today. I felt well, had no problems with the rifle or the bullets. That helped my performance."
Martynov outscored second-place Lionel Cox by 4.3 points. It was the first medal at a major competition for the Belgian shooter.
Rajmond Debevec of Slovenia, the 2000 Olympic champion in 50M rifle three positions, trailed Cox by 0.2 to take bronze.
Cory Niefer of Saskatoon missed the final after finishing 38th in qualifying with 589 points.
Several expected close challengers of Martynov failed to qualify for the final.
Second-ranked Wang Weiyi of China and No. 3 Marco De Nicolo of Italy were four and six points off the qualification limit of 595.
Defending champion Artur Ayvazyan of Ukraine also went out after shooting 593.
Michael McPhail of the United States shot 595 to share fourth place but missed out on a final spot during a shoot-out with nine competitors who all finished on the same score.
Fellow American Matthew Emmons, who won gold in 2004 and silver four years later, did not make the American team.