The Tesla and SpaceX businessman hit the top spot after the former's share price increased this week, which takes him past Amazon founder Jeff Bezos, who had topped the rankings since 2017.
However, Musk's electric car company has surged in value and hit a market value of $700 billion (£516 billion) for the first time.
Alerted to the news on Twitter, he simply replied: "How strange... Well, back to work..."
An older tweet pinned to the top of his social media feed gives some insight into his approach to personal wealth.
He wrote: "About half my money is intended to help problems on Earth, and half to help establish a self-sustaining city on Mars to ensure continuation of life (of all species) in case Earth gets hit by a meteor like the dinosaurs or WW3 happens and we destroy ourselves."
Meanwhile, his fortunes have received a boost in the US where the Democrats will take control of the US Senate.
AS reported by the BBC, Wedbush Securities analyst Daniel Ives said: "A Blue Senate is very bullish and a potential 'game changer' for Tesla and the overall electric vehicle sector, with a more green-driven agenda now certainly in the cards for the next few years."