The business magazine has reappraised the Republican White House hopeful's personal wealth and found it has haemorrhaged $800m since 2015.
Forbes now estimates the real estate magnate's net worth at $3.7bn (£2.7bn).
The publication said the decrease was mainly due to the "softening" of the New York property market.
Mr Trump, who once wrote a book called Midas Touch, has said the country needs a dealmaker-in-chief in the White House.
