The latest estimate - which knocks Trump's ranking to 544th place among the world's richest individuals, from 324th a year ago - is also down $200 million from October, when Forbes had pegged Trump's fortune at $3.7 billion.
According to the U.S. publication's latest global rich list, Gates grew his wealth by US$11 billion during 2016 to US$86 billion, ranking as the richest person in the world for 18 of the past 23 years.
Forbes has tracked Trump's net worth since 1982 and past year he attained a personal high on the list with a ranking of 121.
Former New York City Mayor Michael Bloomberg makes Forbes' list at 10th place with a net worth of $47.5 billion. Besoz showed the biggest increase, with his net worth shooting up from $27.6 billion from past year. Facebook creator Mark Zuckerberg was number five and Oracle co-founder Larry Ellison was number seven.
2,043 billionaires (up from 1,810 in 2016) made the list, with an average net worth of $3.75 billion.
Amancio Ortega: Ortega co-founded the hugely successful high street Spanish retail chain Zara, and now boasts a net worth of $74 billion.
Jeff Bezos: The CEO and founder of e-commerce giant Amazon, Bezos' current net worth is $73.8 billion. The magazine attributes Trump's drop to flagging real estate values in midtown Manhattan. Gates' fortune is listed at $86 billion, and Zuckerberg's at $56 billion.
Amancio Ortega, the Spanish business tycoon best known for owning the Zara fashion label, came in fourth, making him the highest ranking non-American with an estimated fortunate of $71.3bn.
Rising oil prices are also cited another key reason for the rise in the fortunes of some other of the wealthy people on the Forbes list.
Sall stands No. 367 among the world's richest, the new report says. The only other woman in the top 20 was Alice Walton, only daughter of Wal-Mart Stores founder Sam Walton. He was in the third position previous year with $60.8 billion.
Regionally, Asia-Pacific boasted the most billionaires, with 720 billionaires, followed by the United States with 565 and Europe with 530.