All the talk beforehand was of Bolt's injury worries and how his title was under threat from compatriot Yohan Blake.
But, despite a sluggish start, he picked up in the middle of the race to power home in an Olympic record of 9.63 - the second-fastest time ever - with Blake second in a personal best-equalling 9.75.
USA's Justin Gatlin came third, also in a PB, in 9.79.
'It means a lot because there were a lot of people doubting me. It was great to come out and show I am still number one, I am still the best,' Bolt said. 'I've said it from the start, people can talk, all they can do is talk.
'I tell you people that when it comes to the championships it's all about business to me, and I brought it.
'The trials woke me up. Yohan gave me a wake-up call. He knocked on my door and said "Usain, this is an Olympic year, wake up", so I am grateful for that moment because after that I got my head together, got my head in the game.
'I have to give thanks to Blake. He always pushes me. He's worked harder than me. But when it comes to business I do what I have to do. I have a great talent.
'I think he's going to do better next time. Everyone who was in this race, he beat almost everybody. He's going to be more confident and I know he'll do better next time.
'This gold means I am one step closer to being a legend, so I'm working towards that. That's just one step, I have the 200m to go so I'm looking forward to that.'
USA's Sanya Richards-Ross took gold in the women's 400m in 49.55, with Great Britain's defending champion Christine Ohuruogu clocking a season's best of 49.70, just 0.02 ahead of DeeDee Trotter on the line.
Kenya's Ezekiel Kemboi won gold in the men's 3,000m Steeplechase, Kazakhstan's Olga Rypakova triumphed in the women's Triple Jump and Hungary's Krisztian Pars won the men's Hammer Throw.
Ethiopia's Tiki Gelana revelled in the rain as she claimed a surprise gold in the Marathon.