Katinka Hosszu of Hungary made her second top of the podium finish at the Rio Olympics on Monday as she won the women's Olympic 100 metres backstroke.

The others who grabbed gold were China's Sun Yang in the men's 200 metres freestyle, United States Ryan Murphy in the men's 100 metres backstroke and Lilly King in the women's 100 metres breaststroke.

Australian world champion Emily Seebohm, swimming in an outside lane, set the early pace but Hosszu powered her way through the pool in the last half of the race to touch the wall in 58.45 seconds.

It was a second gold in Rio for Hosszu, the self-styled 'Iron Lady' of swimming, after her world record-breaking victory in the 400 individual medley (IM) on Saturday.

"It's really cool, I can't believe I've won the 100 back," the 27-year-old told reporters. "I know I've done the 100 back in the 400 IM a million times and the goal was to make it like second nature when I came to Rio, which is how it felt."

Hosszu, who is also swimming in the 200m backstroke, butterfly and medley, had come away empty-handed from three previous Olympics, despite winning five world championship titles.

Meanwhile, Sun Yang became the first Chinese to win a 200m gold as he completed a golden Olympic treble.

Sun, who won the 400 and 1500 freestyle at the London Games, touched the wall in one minute and 44.65 seconds to beat South Africa's Chad Le Clos by 0.55 seconds.

In his delight at winning, Sun took off his cap and attempted to throw it to Chinese supporters in the crowd above him, but it landed in the pool and he had to retrieve it.

Britain's world champion James Guy, whose room-mate Adam Peaty stormed to breaststroke gold on Sunday and ended his country's 28 year wait for a male Olympic swimming champion, finished fourth.

Ryan Murphy and Lilly King extended the United States swimmers' run of domination in the 100m breast stroke event to six successive Games dating back to Atlanta in 1996 as they won the men's 100 metres backstroke and women's 100 metres breaststroke respectively.

Murphy's winning time of 51.97 seconds was the second fastest in history, just outside the world record of 51.94 set by compatriot Aaron Peirsol in 2009 when non-textile bodysuits were allowed, and an Olympic record.

"I don't even feel tired just because I'm so excited. This means everything to me. I've been swimming for 16 years and to have it come to this is just a dream come true," said 21-year-old Murphy, a first-time Olympian.

Australia's world champion Mitch Larkin had led at the turn, 0.17 of a seconds inside world record pace, with Murphy only fourth.

But the Australian's challenge faded as Murphy powered through. At the touch, Plummer clocked out at 52.40 and denied Larkin a medal by just three hundredths of a second.

Lilly King meanwhile, denied Russia's Yulia Efimova who was once again greeted with resounding boos from the crowd in the women's 100 metres breaststroke.

King swam a strong first 50 metres and led at the turn, with Efimova well back in fourth place. The Russian surged forward in the second half of the race and appeared to be catching her rival, but King finished powerfully with an Olympic record time of one minute and 4.93 seconds, with Efimova 0.57 seconds behind.

"It just proves that you can compete clean and still come out on top," King said.  Before leaving the poolside, she patted the Russian on the back.

Efimova, who after months of uncertainty learned only last Friday that she could compete, broke down after the race in front of reporters, crying into the arms of a friend.

"I can't say that I'm happy, it's just good that I was able to compete," she said.

The Russian said she had wanted to win gold to prove her critics wrong.

"That's why I'm so upset that I didn't manage it, I knew that lots of people were counting on me."