Denmark's Pernille Blume beat the favourites to win the 50 metres freestyle in 24.07 seconds on Saturday and give her country a first Olympic swimming gold medal since 1948.
Simone Manuel of the United States, who won 100 freestyle gold in a dead-heat with Canada's Penny Oleksiak, was just 0.02 behind for silver while Aliaksandra Herasimenia of Belarus took the bronze in 24.11.
Ranomi Kromowidjojo, the 2012 Olympic champion from the Netherlands, finished sixth while Australia's Campbell sisters Cate and Bronte again failed to medal in an individual sprint.
Britain's Fran Halsall finished fourth in 24.13.
'It's been completely unreal for me'
The gold was Denmark's third in swimming, and Blume's first of any colour at an Olympics. The other two were won by Karen-Margrete Harup (100m backstroke) and Greta Anderson (100m freestyle) at the 1948 London Games.
But Blume was more excited about the last 48 hours.
"Its been completely unreal for me," she said.
"The gold medal means so much, but also to the team because we had such an amazing energy in the call room," she told reporters after anchoring Denmark to a bronze medal in the 4x100 medley relay about 45 minutes after the 50 freestyle.
"It has just been four years of hard work ... we did a really good job as a team," added the 22-year-old, who took a break from swimming about eight months ago to "figure out how to enjoy what I'm doing most.
"It was really important for me to take some time out and just clear my head and come back in the pool with new energy. I'm so happy that I actually came back in the pool. It's been an incredible journey."
Denmark's last previous Olympic swimming medal was Lotte Friis's bronze in 800 freestyle at the 2008 Beijing Olympics.
Blume's time was 0.02 off Kromowidjojo's Olympic record, set in London four years ago, and the second fastest in the event this year.
Faced an intimidating array of rivals
She had been fastest qualifier in the heats and semi-finals but faced an intimidating array of rivals on the blocks before the quick thrash down the pool.
Cate Campbell, the 100 freestyle world record holder, swam 23.84 in Adelaide in April but again failed to hit top form in the final individual women's event of the competition.
She and Bronte, the world champion for the 50 and 100 freestyle, won gold together in the 4x100 freestyle relay on the opening Saturday but will go home without individual titles.
"It's been a tough week, there is no denying," said Cate, speaking after ending the meet with a silver in the 4x100 medley.
"The 50 is kind of a lottery and it just hasn't been my week obviously. My number wasn't up this time. I couldn't be too disappointed. It's 0.3 off my best but I've only swum that time once, and when I was in amazing form.
"I executed a good race and I can't be too hard on myself."