Forbes estimates Mr Trump's worth collapsed from $US3.5 billion ($4.4 billion) to $US3.1 billion ($3.9 billion) this year, with the magazine saying it was partially due to a fall in NY property values and a dip in revenues at his golf courses.
Unlike Forbes, which made its calculations based partly on stock wealth on February 9 this year, Bloomberg updates its billionaires list every day.
A record 2,208 billionaires feature on the magazine's 32nd annual ranking of the world's billionaires.
The richest person in Europe and fourth on the list is the boss of luxury goods firm LVMH, Bernard Arnault, who has a fortune of $US72bn.
U.S. investment superstar Warren Buffett, who is worth $US84bn, is third.
Forbes' list contains 259 newcomers, including the first-ever cryptocurrency billionaires.
Forbes came out with their Billionaires 2018 list, and for the first time this list will not include any Saudis.
He's now worth an estimated $112 billion, up $39.2 billion from 2017.
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.
At No. 5 is Facebook's Mark Zuckerberg, with an estimated $71 billion.