Jamaica's Usain Bolt became the first athlete to win three Olympic 100m titles by beating American Justin Gatlin to gold at Rio 2016.
Bolt, 29, ran 9.81 seconds in his final Olympics to replicate his success at Beijing 2008 and London 2012.
Gatlin, twice banned for doping offences, finished 0.08 seconds behind Bolt to take silver.
"Somebody said I can become immortal. Two more medals to go and I can sign off. Immortal," said Bolt.
Canada's Andre de Grasse took bronze in a personal best of 9.91, ahead of Bolt's Jamaican team-mate Yohan Blake.
There was no British interest in Sunday's showpiece as CJ Ujah and James Dasaolu were eliminated in the semi-finals.
Bolt remains on target to leave Rio with a third successive Olympic treble, and a ninth gold, having won the 100m, 200m and 4x100m relay titles in 2008 and 2012.
"It wasn't perfect today but I got it done and I'm pretty proud of what I've achieved - nobody else has done it or even attempted it," he added.
"I expected to go faster, but I'm happy that I won. I did what I had to."
Bolt was slower out of the blocks than 34-year-old Gatlin, who was aiming to regain the title he won at Athens 2004.
But he surged through from 60 metres to pass Gatlin and comfortably win his seventh Olympic gold.
Bolt received a hero's reception as he walked out into Rio's Olympic Stadium before the race, and the crowd chanted his name after his victory.
"It wasn't about the time, it was just about winning the gold and going out on top," said Michael Johnson, four-time Olympic champion and BBC Sport analyst.
The world record holder, who said in February he would retire from athletics after the 2017 World Championships, competes in qualifying for the men's 200 on Tuesday, with the 4x100m relay beginning on Friday.
Bolt's major 100m wins
2008 Olympic Games, Beijing
9.69 - world record
2009 World Championships, Berlin
9.58 - world record
2012 Olympic Games, London
9.63 - Olympic record
2013 World Championships, Moscow
2015 World Championships, Beijing
2016 Olympic Games, Rio
Bolt's a unique individual - Michael Johnson
Bolt showed an expectant Rio crowd he was in great shape by clocking a season's best 9.86 as he cantered to victory in his semi-final.
And the sport's greatest showman produced an even better run when it really mattered to send the Olympic Stadium into raptures.
"After the semi-final I felt extremely good," Bolt added.
"I wanted to run faster but with the turnaround time, we normally have two hours, but we had one hour 20 minutes, it was challenging.
"This is what we train for. I told you guys I was going to do it. Stay tuned, two more to go."
Michael Johnson, four-time Olympic gold medallist:
"That was a fantastic performance by Usain Bolt, he slowed down and said 'I've got this'.
"I didn't expect the race to unfold the way it did. Gatlin got a great start but it was always within Usain Bolt's reach.
"Gatlin knew that this was not going to happen. He was never going to be able to beat a healthy Usain Bolt.
"He has been an amazing ambassador for this sport and in creating a brand for himself and the Jamaican athletes."
Steve Cram, BBC athletics commentator:
"He still looks like he really enjoys this. This is what he does. He gathers titles like daisies in a field.
"Our sport has a lot of critics and it's going through dark times but we should not forget that this is what can be done. This is what can be achieved.
"He is almost God-like."
Booed Gatlin comes up short again
Gatlin is a divisive figure in the sport having twice been banned for doping offences and, in stark contrast to Bolt's reception, walked out to the start line to a chorus of boos.
Bolt admitted afterwards he was "surprised" by the crowd's reaction, but Gatlin himself was unmoved.
"I get so much respect from my own competitors, all of them wish me well," the American told BBC Radio 5 live.
"When you have people out here - who have never met me before - coming to boo me, I just say: get to know who I am. I have worked hard to get on the podium. I'm honoured to be here for my country."
Gatlin's first drugs ban in 2001 was reduced from two years to one after he proved the amphetamines he was taking were for an attention deficit disorder.
He then tested positive for testosterone in 2006, a year after winning the 100m and 200m double at the World Championships.
The American served a four-year ban that was twice reduced, first from a lifetime then to eight years.
Gatlin returned to the track in 2010, claiming Olympic bronze at London 2012 and losing to Bolt in the 2015 World Championships.
He was the clear favourite to win in Beijing last year, and many thought he could beat Bolt in Rio.
Gatlin had recorded the fastest time of the year, clocking 9.80 last month, while Bolt injured a hamstring at the end of June.