Some of the wealthiest people in the world are professional investors. Billionaires like Warren Buffett, Ray Dalio, Bill Ackman, and Ken Griffin have made their fortune by getting others to invest with them and making smart investments.

However, while many of them are regarded as financial wizards, often their investments are utterly pedestrian. In fact, a number of billionaire investors count S&P 500 index funds among their top holdings. Among those are Buffett's Berkshire Hathaway, Dalio's Bridgewater, and Griffin's Citadel.

An S&P 500 exchange-traded fund (ETF) is the easiest way to get exposure to the broad market. You can simply buy and hold one investment that will track with the S&P 500, an index of 500 large-cap U.S. stocks, that is often regarded as "the stock market" even though it doesn't include every publicly traded company.

It's easy to see why S&P 500 index funds are so popular with the billionaire investor class. The S&P 500 has a long history of delivering strong returns, averaging 9% annually over 150 years. In other words, it's hard to find an investment with a better track record than the U.S. stock market.

It's also an easy investment to own. The S&P 500 is typically the benchmark that hedge funds try to beat, but no one will look foolish owning an S&P 500 ETF, and it's a good place to park your money until you have a better idea.

A person typing on a computer with the word ETF overlaid on it.

Image source: Getty Images.

The billionaires that own the S&P 500

Warren Buffett is probably the most famous investor in the world, and he's also a big advocate of the S&P 500, saying it's always smart to bet on America. In fact, he's requested that 90% of his personal wealth be put in an S&P 500 index fund when he dies.

Perhaps, it's not a big surprise then that Berkshire Hathaway owns S&P 500 index funds. Buffett's conglomerate owns both the Vanguard S&P 500 ETF (VOO 0.78%) and the SPDR S&P 500 ETF (SPY 0.77%), owning nearly $17 million of each. Each one represents a paltry 0.01% of Berkshire's stock portfolio, but it's still no accident that they're there. Berkshire has owned them since 2019.

Ray Dalio's Bridgewater Associates is another billionaire-backed hedge fund that owns the S&P 500. Bridgewater's second-biggest holding is the iShares Core S&P 500 ETF (IVV 0.78%) with $878 million invested; the SPDR S&P 500 ETF is also a top-10 holding, making up $426 million of the portfolio.

Ken Griffin's Citadel Advisors is another big backer of the S&P 500. It counts the SPDR S&P 500 ETF as its third-biggest stock holding with just over $1 billion invested as of the end of the third quarter, and it also owns the Vanguard S&P 500 Fund. Citadel first bought the SPDR fund in 2014, and added nearly $400 million more to the ETF in the third quarter.

Is the S&P 500 ETF right for you?

There's another reason an S&P 500 index fund might be such a popular choice right now for even the top investors. There's a lot of uncertainty in the market these days.

Some investors think the economy is headed for a recession, as JPMorgan Chase CEO Jamie Dimon recently warned. Others believe that a new bull market has begun and that interest rates will soon fall, which is bullish for stocks. Bridgewater's Dalio even said that cash was worth holding, which helps maximize flexibility, after formerly calling it "trash."

No one knows for sure where the market is headed this year, but owning the S&P 500 over the long term has been a smart move for more than 100 years. Buying one of these ETFs is about the easiest move you can make as an investor, and it could be the smartest one as well. After all, as you can see from the list above, even billionaires count on the S&P 500 to build wealth.