The magazine, which puts out annual lists of the world's richest people, blamed Gates' decline to the slide in Microsoft shares from the day before the company announced a $44.6 billion offer to buy Yahoo, Jan. 31, to the day Forbes calculated stock prices into its rich list valuations, Feb. 11.
Had Microsoft shares not declined so much, Gates would have been in a close race with investing mogul Warren Buffett for the top spot on the list, Forbes said.
Buffett took over as the world's richest man this year with an estimated $62 billion fortune, while Mexican communications industry leader Carlos Slim Helu came in second with $60 billion, Forbes said. Gates' fortune was valued at $58 billion.
Microsoft helped put Mark Zuckerberg, the youngest ever billionaire on Forbes' list. The founder of popular social networking Web site Facebook is worth $1.5 billion, according to the magazine, based on a calculation involving Microsoft's $240 million investment last year for a 1.6 percent stake in Facebook.
Zuckerberg ranked 785th overall on the Forbes list.
Other American technology industry leaders toppled down the rich list. Larry Ellison, CEO and founder of Oracle, fell to 14th place on the Forbes list, from 11th last year, while Microsoft co-founder Paul Allen tumbled to 41st place from 19th place last year.
Google founders Sergey Brin and Larry Page retained their titles as the richest young people on the Forbes list, coming in at $18.7 billion and $18.6 billion, respectively. They ranked 32nd and 33rd overall.
The remaining top 10 richest people in the world are mainly in heavy industry and commodities, according to Forbes. The next three richest men are all from India. Fourth place, Lakshmi Mittal, is a steel magnate, while fifth place Mukesh Ambani is in petrochemicals and his brother, sixth place Anil Ambani, is in power and communications. Oleg Deripaska of Russia made much of his fortune in aluminum, putting him in the ninth spot.
Retail titans took two of the top 10 spots. Ikea founder Ingvar Kamprad of Sweden took seventh place on the list, while the founder of German discount store Aldi, Karl Albrecht, took tenth. Indian real-estate developer KP Singh came in eighth on the list.