Her score of 15.933 confirmed she had beaten team-mate Aly Raisman by 2.100 points after a demonstration of jaw-dropping skills across four apparatus.
Biles has a remarkable record since her debut at the 2013 World Artistic Gymnastics Championships in Antwerp.
How Simone Biles transcended gymnastics to become a true Olympic great
In fact, team-mate Raisman went as far to say that Biles is bordering unbeatable.
"No one goes into this thinking they can beat Simone. I'm sure most people don't go into it thinking they can beat Usain Bolt either. So it's kind of the same thing," said Raisman after he event.
However, the athlete herself dismissed comparisons to other Olympians.
"I'm not the next Usain Bolt or Michael Phelps, I'm the first Simone Biles. To me, I'm just the same Simone. I just have two Olympic Gold medals now. I feel like I did my job tonight," said the 19-year-old.
Biles' triumph made her the fourth successive American to win the title -- following the successes of Carly Patterson, Nastia Liukin and Gabby Douglas -- which is the longest winning streak by a country in the Olympic discipline.
With additional reporting from Reuters
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