The 44-year-old, who serves in the air force at home, scored 705.5 to finish ahead of Belgium's Lionel Cox on 701.2 with Slovenia's Rajmond Debevec third on 701.
The mark bettered that of previous world record holder Germany's Christian Klees, who scored 704.8 at the 1996 Atlanta Games.
"I am absolutely delighted. This will mean everything to my country," Martynov said through an interpreter, before going off to smoke a cigarette.
"I have been over 15 years in this discipline and it is one of the best feelings."
The Belarussian was using a 1999 rifle and rounds that were made in 1985, a combination that brought him bronze medals in the discipline at the 2000 Sydney and 2004 Athens Games.
"A rifle isn't a wife but you have to look after it and give it tender, loving care," Martynov said of his prized possesion.
"I'm not exactly prejudice against any recently made rifles or rounds it is just when you choose you choose something that feels more comfortable," Martynov said.
"I have a (new) rifle back at home but when you are getting ready for the Olympic Games it is too late to change anything so I am yet to lay my hands on it."
The stone-faced Belarussian could probably give the new gun away, such was his precision on Friday.
He equalled the world record in the 60-shot qualifiers where he scored a maximum 600 to finish one point ahead of Cox.
Martynov then started the eight-man final, where shooters fire 10 shots aiming for a maximum score or 10.9 with each, strongly as he extended his lead over Cox to 3.2 after six rounds.
With a lowest score of 10.2 in the final, Martynov showed a consistency that could not be matched by his opponents. He shot a maximum 10.9 in the ninth round to bring the crowd to their feet and afford himself the luxury of a massive 4.1 lead ahead of the finale.
After firing a 10.6 with his last shot, Martynov cracked a shy smile and punched the air in delight in front of a packed crowd in the indoor range at the Royal Artillery Barracks.
Cox fired a 10.4 with his final shot to claim the silver and hold off a charge from 49-year-old Debevec, who had started the final three points behind the Belgian.
The 31-year-old Belgian was surprised as anyone to be standing on the podium.
"I will be back to my job as a public service inspector in two weeks' time. I work full-time but I will stay until the end of the Games," he told reporters.
"I did not expect to make the final so I have not made any plans. I don't know if I will do anything today but I am sure I will celebrate."
Great Britain’s Jonathan Hammond, meanwhile, is pledging to learn from his Olympic experience after his London 2012 dream ended with failure to make the final.
Hammond recorded a score of 593 and missed the shoot-off by two points as he finished in 17th place while team-mate James Huckle also failed to make the eight-man final, ending in 29th place with 591.
And 31-year-old Aberdeen-born Hammond, who finished 34th in Beijing, admits that while he was content with his score at the Royal Artillery Barracks, he could have performed better.
He said: "For the conditions it's not a bad score. It was really tricky out there between the whole atmosphere and the environment and the wind. It's a solid score and I'm fairly happy with my performance.
"I probably made a few mistakes in there and got the wind wrong a couple of times, but I'm only two points away from the final. It's all part of the learning curve and something I can build on."
The British pair failed to emulate the heroics of team-mate Peter Wilson, who won the gold medal in yesterday’s double trap final, as they failed to secure the eighth place needed to progress.
And 21-year-old Huckle believes that he will learn lessons from his London 2012 campaign as he admitted he lost the momentum after a great start.
He said: "Conditions-wise, I think it was one of the better days I've seen on the range for the wind. I knew at the start of the match there were going to be some pretty good scores.
"I started off with a 100. I thought it was a great way to start. Then there were just a few shots where it didn't go quite where I thought it would.
“I could feel my technique slipping a little bit and I was like, that's fine, just stay calm. Unfortunately you make a few mistakes at this level and you get punished pretty quickly.”