Trump was ranked at No 544 on the magazine's annual richlist, with a net worth of around $3.5bn.
Forbes said that falling house prices in Manhattan was responsible for the president's dwindling fortunes.
Mr Trump also spent millions of dollars on his campaign, which he claims to have self-funded, though this is fiercely disputed. Dan Alexander, an associate editor at Forbes Media said
“Although Donald Trump’s name is plastered on buildings all around the world, his core assets are still located in a small part of New York City,"Forty percent of Donald Trump's fortune is tied up in Trump Tower and eight buildings within one mile of it," Lately, the neighborhood has been struggling (relatively speaking)."
"Profits are down at Trump Tower, and the overall neighbourhood surrounding has been struggling over the last year. Since Trump owns so many buildings in such a small area—including Trump Tower, Trump International Hotel & Tower, NikeTown and 1290 Avenue of the Americas—the softening of the market has impacted his net worth more than anything else over the last year.”In October 2016, Forbes had put Mr Trump's wealth at around $4.5bn, placing him at No 324 on the list.
For the fourth time in a row, Bill Gates was named as the richest man in the world, with an estimated fortune of $86bn, while Warren Buffett came in at second place with a net worth of $75.6bn.
Amazon's chief executive Jess Bezos came in third place, with 72$bn - up from "27bn the previous year, making him the biggest wealth gainer on the list.