The victory represents Biles' 16th gold medal at the worlds as she added to her all-around world titles she won in 2013, 2014, 2015 and 2018.
Needing 12.301 in floor exercise to reclaim her title in the final rotation, Biles scored 14.400 points to finish with an overall score of 58.999 -- beating China's Tang Xijing by a huge margin of 2.100.
Gymnastics whistleblower Downie dropped from Olympic team
The 22-year-old Biles is now just one short of Belarusian Vitaly Scherbo's record haul of 23 medals - a milestone she could surpass by the end of the individual events at the championships this weekend.
Tang finished with a score of 56.899 to claim silver on her championships debut, while Russia's Angelina Melnikova took bronze with 56.399.
Tweddle and Liukin inspiring hotshot Kinsella to Olympic glory
Hall: Olympic debut caps remarkable turnaround after 'directionless' days