The Tesla and SpaceX entrepreneur was pushed into the top slot after Tesla's share price increased on Thursday.
He takes the top spot from Amazon founder Jeff Bezos, who had held it since 2017.
Musk's electric car company Tesla has surged in value this year, and hit a market value of $700bn (NZ$964bn) for the first time on Wednesday.
That makes the car company worth more than Toyota, Volkswagen, Hyundai, GM and Ford combined.
Musk reacted to the news in signature style, replying to a Twitter user sharing the news with the remark "how strange".