Jeff Bezos is the richest person in modern history.

The Amazon.com founder's net worth broke US$150 billion ($221.3b) in New York on Monday morning, according to the Bloomberg Billionaires Index.

That's about US$55b more than Microsoft Corp. co-founder Bill Gates, the world's second-richest person.

Bezos, 54, has now topped Gates in inflation-adjusted terms. The US$100b mark that Gates hit briefly in 1999 at the height of the dot-com boom would be worth about US$149b in today's dollars.

That makes the Amazon chief executive officer richer than anyone else on earth since at least 1982, when Forbes published its inaugural wealth ranking.

Bezos crossed the threshold just as Amazon prepares to kick off its 36-hour summer sales event, Prime Day. The company's share price was US$1,825.73 at 11:10 a.m. in New York, extending its 2018 gain to 56 per cent and giving Bezos a US$150.8b fortune.

His net worth has soared by US$52b this year, which is more than the entire fortune of Mukesh Ambani, the newly crowned richest person in Asia. It also puts Bezos's personal fortune within spitting distance of the Walton family's US$151.5b, which is the world's richest dynasty.

"It's hard to even put it in perspective," said Michael Cole, CEO of Cresset Family Office. "It's such a staggering number."

A Federal Reserve report found the top 1 per cent of US families controlled 38.6 per cent of wealth in the US in 2016, compared with 22.8 per cent held by the bottom 90 per cent.

Last year, Oxfam International found that more than 80 per cent of earnings went to the top 1 per cent of the world population.

Behind Bezos on the Bloomberg index is Gates, with a US$95.5b fortune followed by Warren Buffett with US$83b.

Gates would have had a net worth of more than US$150b if he'd held onto assets that he's given away, largely to the Bill & Melinda Gates Foundation. He's donated almost 700 million Microsoft shares and US$2.9b of cash and other assets since 1996, according to an analysis of his publicly disclosed giving.