The 19-year-old dominated the floor with the highest difficulty and execution scores, winning with 15.966 points on Tuesday, reports Xinhua.
"When I was walking up my legs felt like rocks, but I've done floor routines like that before so I wasn't nervous about it," said Biles after taking part in her fifth final at Rio with tired legs.
The first-time Olympian also won the women's team, all-around and vault events.
The 10-time world champion also reached the balance beam final as the top qualifier, but made a mistake after a front tuck so as to finish third at last.
"Your first Olympics you walk away with five medals, that's not tough at all. Especially four being gold, that's just unheard of. I'm very proud," Biles said.
Defending champion Alexandra Raisman failed to match up with her outstanding teammate Biles, but managed to bring home a silver in 15.500 points.
Britain's Amy Tinkler finished third in 14.933 points.
Chinese rookie Wang Yan finished fifth in the event, which means that the Chinese gymnastics team will have to leave Rio only with two bronze medals from the team finals.
It's the first time that Chinese gymnastics failed to win gold at the Olympic Games.
Earlier on Tuesday, China's world champion You Hao fell off in landing after performing his parallel bars routine with the highest difficulty score of 7.4 points.
The 24-year-old sat on the mattress for seconds. "Because I felt so embarrassed," he said.
Oleg Verniaiev of Ukraine, who took the all-around silver on last Wednesday, won the parallel bars final in 16.041 points.
US gymnast Danell Leyva took the silver in 15.900 points, followed by David Belyavskiy of Russia in 15.783.
Besides, Fabian Hambuechen of Germany completed a full set of medals at the Olympics after winning the high bar on Tuesday.
Hambuechen, 28, was out first in the final and his score of 15.766 points would be unbeatable.
"It is indescribable. I can't realize what happened today," said the high bar specialist who took a bronze at the Beijing Olympics and grabbed a silver in London four years ago.
"Before the Games I said this was going to be my last competition. I am going to retire, and to finish like that is a dream comes true."
US gymnast Danell Leyva took the silver in 15.500 points and Nile Wilson of Britain finished third in 15.466.
The US team led the medal table with four golds, six silvers and two bronzes. Britain placed second with 2-1-3 and Japan third with 2-0-1.
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