His Jamaican compatriot, Yohan Blake, unbeaten in over a year and victorious over Bolt in their national trials, was second in 9.75. Justin Gatlin of the United States was third, in 9.79. The wind blew at 1.5 metres per second at their backs. It was the greatest 100m final ever, with seven men under 10 seconds.
Somebody threw a bottle on to the track behind the blocks, but it did not distract anyone as they got away first time. Bolt showed no sign of stress, despite disqualification a year ago in the World final which was won by Blake.
Those long levers – Bolt is 6ft 5in – mean he is not the fastest out the blocks. He also dislikes the new-style starting blocks, ill-suited to his size-13 spikes. He admitted later that it was a problem, and cost him almost a metre, but once into his running, there was only one winner.
This result reaffirmed his position in the pantheon of sprinters. He is only the second man to retain the 100m title, after Carl Lewis, in 1984 and '88.
Gatlin, the 2004 champion in Athens is the first drug-cheat sprinter to return and win a medal. The US were third, fourth, and fifth, suggesting they will pose a threat to Jamaican hopes of retaining the relay crown, especially with former world record holder Asafa Powell pulling up close to home with a groin injury. Powell was the only man not to break 10 seconds. He was well placed to do so, but limped home in 11.99.
"I injured myself," he said. "I just couldn't push. I tried in the semi-final to release the tension. I got out very well and stumbled. I knew I would be up there with the medals. I need to fix this. I knew they were ready from the semifinal. I knew it was going to be a Jamaican one-two."
Bolt confirmed worries about his start: "I slipped a little in the blocks. I don't have the best reactions, but I secured it, and that's the key.
"My coach told me to stop worrying about the start and concentrate on the end, because that's my best."
Gatlin led for some 50 metres, but Bolt is an unstoppable force. At 60m he was clear, and Blake came through to nail Gatlin convincingly.
The American, who served four years out, returning in 2010, said: "It feels great to be back after eight years. Simply just to come back and use my God-given talent. I went out there and I did the best I could do."
Gay (9.80), Ryan Bailey (9.88) and Churandy Martina (9.94) were the fastest fourth, fifth, and sixth placers in Olympic history.
Gay, former World 100 and 200m champion and second quickest ever at the distance, expects to face Bolt again in the 200. The prospect clearly daunts him, and he was in tears. "I am going to try. I just want to be in the final with him, and try to go. It's tough, but I have no excuses. I gave my all. Everybody came together and made me the best I can be. I tried my best."
Bolt paid tribute to the crowd, and played to them. "It was wonderful," he said. "I knew it was going to be like this."
He added that training partner Blake "works harder than me, but I knew what I needed to do, and I have great talent. He will do better next time because he was a little bit stressed this time."
The pair wrapped themselves in the golden flag of Jamaica, hugged, and took a lap of honour together, engulfed by photographers. Bolt danced and did a somersault in front of the main stand. On completing his circuit, he struck his lightning bolt pose to chants of: "Usain! Usain!"
Blake, nicknamed The Beast for his work ethic, said: "He's the fastest man in the world, and I've got a silver medal. What more can I ask for? To be the second fastest man in the world behind Bolt is an honour."
Two million people applied for tickets for last night, paying up to £750 each. But domestic interest ended in the semi-finals where seven qualified on 9.96 or better. This was too much for the home challenge. Dwain Chambers, thrice a world finalist and best equipped to reach the final, had the misfortune to be drawn in the hottest semi, next lane to Bolt. He seemed to lose focus when the Jamaican pulled clear. The writing was already on the wall as Bolt finished on 9.87, throttling back some 14 metres out. Chambers, fourth in 10.05, finished tenth overall.
His ritual is always to look at his watch before settling in the blocks. At 34, his time surely seems over. He accepted the result with grace: "I'm happy, but part of me is disappointed as I wanted to make the final. Based on my time yesterday [10.02], that would have got me through. I'm happy to be here. I've never smiled so much in my life. We've got the relay next. Adam [Gemili] and I will keep ourselves motivated for that, but we'll probably have a McDonalds tonight. And then we'll get ourselves focussed for the relay."
Newly-crowned World Junior champion Gemili was introduced as: "the sprinting find of 2012". The smile that split his face lit up the stadium screen.
Drawn next to world champion Blake, with Gay a lane away on the other side, he was not overawed.
He finished third, and eleventh fastest overall. A big future beckons for the former Thurrock footballer. James Daaolu was seventh in his heat (10.18), eleventh fastest.
Gemili said he would "stick with athletics for now, because "it's all going really, really well."
Jamaica's victory cemented the sprint power shift from the USA. The calypso parties and barbecues started a day earlier when Shelly-Ann Fraser-Pryce won the women's 100m, retaining the title she won in Beijing with 10.75, ahead of Carmelita Jeter of the US (10.78) with Jamaica's Veronica Campbell-Brown taking bronze in 10.81.
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