Biles had 57.491 points at the end of the all-around finals of the 48th Artistic Gymnastics World Championships 2018 at Aspire Dome in the Qatari capital, Doha.
"This one has probably been the hardest to get out of all my World Championships and Olympic medals, and the scariest one," the 21-year-old Biles said, reports Efe.
"It's exciting to bring back a gold medal for the US," she added.
Meanwhile, Mai Murakami became the first Japanese gymnast to claim an all-round silver medal. The defending champion, 17-year-old Hurd Morgan of the US, won bronze.
"It is very important to cheer for each other," Biles said, referring to Morgan.
"We have been training for this competition for so long. It's very exciting to be out there with her, and we're excited that we brought hardware back."
Earlier this, US Gymnastics said that doctors found a kidney stone when Biles underwent medical checks at a hospital emergency room.
The event in Doha was first international competitive appearance for Biles since her participation in the 2016 Olympics in Rio de Janeiro, where she won four gold medals -- a US record for women's gymnastics at a single Olympics.