He topped the same list past year and held the title on the Forbes 400 ranking released in October, when his net worth was $8.5 billion.
Villar, meanwhile, doubled his estimated net worth to $3 billion from last year's estimate of $1.5 billion.
Top of the list is the founder and boss of online giant Amazon, Jeff Bezos.
Bezos' fortune has leapt to $112bn from an estimated $39.2bn past year, thanks in the main to the soaring Amazon share price. He has said he plans to sell $1 billion a year in Amazon stock to fund the company.
Most of his fortune is tied to Amazon shares which have risen from $846 a share last March to $1,523 per share last night.
Forbes' 96th-richest Robert Kuok is oldest billionaire in top 100 list
Trump dropped more than 200 spots from 544th richest in 2017 to 766th in 2018 on the magazine's annual Billionaires List as a result.
"Bezos has seen his wealth remain in excess of $100 billion, and with the likes of Bill Gates, Warren Buffett and Mark Zuckerberg also tipped to reach that eye-watering milestone, it seems we're entering the era of the centi-billionaire", Vishal Chhatralia, vice president digital operations for RS Components said in a statement. His fortune is up from $75.6bn a year earlier. In fourth is Bernard Arnault and family, at $72 billion.
Although Trump is part of that elite group, he saw his fortune sink by about $400 million to $3.1 billion during his first year in office.
Bernard Arnault - The first member of the list not to come from the US has a net worth of $72 billion and calls France his home.
Sun Pharmaceuticals founder Dilip Shanghvi ranks next as the fifth riches Indian with $12.8 billion at the 115th global spot.More news: On Janhvi Kapoor's 21st birthday, Sonam Kapoor shares a heartfelt message
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