The magazine probed 28 assets or asset classes owned by the New York businessman and found 18 had declined in value, including Trump Tower in Manhattan and his Mar-a-Lago club in Palm Beach, Florida.
The value of his 40 Wall Street property in downtown New York had also declined, according to Forbes, which said it has been tracking Trump's wealth for 34 years.
At the same time, one asset remained steady while seven others - including 555 California Street, the second tallest building in San Francisco - saw their value increase, it reported.
Representatives for the Trump campaign could not be immediately reached for comment.
The report comes as Trump, known as a reality television star as well as for his real estate business, heads into the final stretch of campaigning for the Nov. 8 election.
Trump, seeking elected office for the first time, has touted his business acumen in pressing for change in Washington. He has vowed to rip up trade deals, negotiate with allies to make them pay more for their U.S. defense protection, and have Mexico pay for a wall he has proposed along the southern U.S. border. If elected, he would be the first billionaire to serve as president, according to Forbes.
Details of Trump's wealth are not publicly known because, bucking a decades-old tradition of U.S. presidential candidates, he has refused to release his tax returns. His Democratic rival, former Secretary of State Hillary Clinton, earned $10.75 million in 2015 alongside her husband, former U.S. President Bill Clinton, their tax filing showed.
Clinton and others, including some prominent Republicans, have pressed Trump to release his returns. They have also raised questions about his net worth as well as his charitable contributions and business ties.
Forbes said its analysis did not place any value on Trump's brand. It also disputed Trump's assertion in Monday night's presidential debate that he reported $694 million in income from last year to the Federal Election Commission. Forbes said Trump mixed revenue and income in the document.
Trump has so far given $54 million to his presidential campaign. His campaign has also paid his companies for office space in Trump Tower and use of his private airline, among other expenses, federal election filings have shown. Politico reported separately on Wednesday that Trump is considering giving more money to his campaign to fund an advertising blitz, citing three sources close to him.
Amid the campaign, Trump made just one real estate deal in the past year: purchasing a Charleston, South Carolina, warehouse that was in foreclosure, Forbes said.
(Additional reporting by Emily Stephenson and Grant Smith; Writing by Susan Heavey; Editing by Frances Kerry)
