Medvedeva finished with 223.86 points overall to add the world crown to her Grand Prix Final and European titles, finishing 8.47 points ahead of Ashley Wagner, who claimed a first podium place in 10 years for an American in the event.
"I won't realize quickly I won today because one year ago, I was still skating juniors," she told reporters through a translator.
"Actually I don't really have any emotions right now... I left everything on the ice."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Fellow Russian Anna Pogorilaya finished third and American champion Gracie Gold, who led after the short program, fell early in a nervous performance to drop down to fourth.
Medvedeva also became the first women to win world junior and senior women's titles in successive years.
Earlier, Canadians Meagan Duhamel and Eric Radford came from behind to successfully defend the pairs title they won in Shanghai 12 months ago.
The Canadian duo had struggled for most of the season since that triumph in China but Duhamel and Radford found their form with a personal-best free skate score of 153.81 points for a 231.99 total.
They skated to Adele's "Hometown Glory" and were the only pair to land a quad throw Salchow. "It was an out-of-body experience," Duhamel said. "How did we just do that? It's like a dream."
China's Sui Wenjing and Han Cong, who led after the short program, struggled with a poor free skate to settle for a silver medal for the second straight year.
The Chinese pair led by 2.67 after the short program but could only manage a score of 143.62 for 224.47 total points. Five-time champion Aliona Savchenko of Germany and new partner Bruno Massot, skating together at a world championships for the first time, took bronze with a 216.17 total.
(Reporting by Tim Wharnsby in Toronto; Editing by John O'Brien)
