South Africa's Cameron van der Burgh set a new world record to win the 100m men's breaststroke final.
He blazed away from the start and could not be caught as he clocked 58.46 seconds.
Australia's Christian Sprenger claimed silver, with Brendan Hansen of the United States in third.
Japan's Kosuke Kitajima was bidding to become the first male swimmer to win the same event at three successive Games but finished fifth.
"Van der Burgh has learnt in the last couple of years to use his 50m speed and make the most if it in his 100m. He was very good, rolling all the way to the finish. As for Hansen, the comeback kid, I was not even sure he would make the team. Great story for the 30-year-old. I think you have three very happy people on the podium."
Van der Burgh, 24, had qualified fastest for the final, when he set the quickest time of 2012 - and an Olympic record - on Saturday.
He continued in the same vein to win the gold, shaving 0.12 seconds off Brenton Rickard's 2009 record.
The victory meant Van der Burgh became the first South African man to win Olympic gold in an individual swimming event.
The 100m breaststroke was the event of Norwegian world champion Alexander Dale Oen, who died of heart failure last April.
"I just have to pay tribute to Alexander Oen," said Van der Burgh. "I know he has been with me this year. He helped me finish the race in such a strong manner.
"Alexander pushed me in training. It made me realise I had to go faster to win gold. That is what we trained for and that is what we have achieved."