The magazine released its annual ranking Tuesday, showing Trump's wealth declining for the second year in a row. No. 4 is chairman and CEO of luxury goods company LVMH, Bernard Arnault, and his family, with $72 billion.
United States investment superstar Warren Buffett, who is worth $US84bn, is third.
The list is published in Forbes's latest edition under the headline "The super-rich continue to get richer, widening the gap between them and everyone else".
The number of women on the list increased from 227 to 256.
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The magazine attributed Trump's decline in wealth to "a tough NY real estate market, particularly for retail locations; a costly lawsuit and an expensive presidential campaign".
Meanwhile, Trump, who became the first billionaire president in history past year, dropped to No. 766 on the list, with a $3.1 billion net worth. Egyptian billionaire Nassef Sawiris with a net worth of $6.6 billion is the world's richest Arab, as Prince Al Waleed Bin Talal was excluded from the list.
Forbes' list contains 259 newcomers, including the first-ever cryptocurrency billionaires.
Despite Mr Trump's previously inaccurate claims he was worth more than $10 billion, Forbes said he is still the richest president in U.S. history. Gates, Zuckerberg, and Buffett are at least a $1 billion richer.