Mr Soros’ investment fund bought about 3.1 million Class A shares in the club, according to the Securities and Exchange Commission.
Class A shares carry less voting power than Class B shares.
His shares equate to a 1.9% stake in the entire club, worth about $40.7m (£25.8m) at Monday’s closing price.
Manchester United floated on the US stock market earlier this month, valuing the club at more than $2.3bn (£1.46bn), making it one of the biggest sports clubs in the world.
But since its 10 August listing, its share price has fallen 6.7%.
Manchester United has been controlled since 2005 by the Glazer family, the billionaire US sports investors who also own the Tampa Bay Buccaneers American football franchise.
About half of the $233m that the club raised from its flotation will go to paying off the club’s debts, with the rest going to the Glazers.
Manchester United’s shares on Monday closed down 2.7% at $13.06, after hitting a fresh low of $12.91 earlier in the day.
The 82-year-old investor, who oversees $25bn in assets through his Soros Fund Management LLC, has in the past eyed other football clubs as lucrative investments.
He considered a takeover of Italian club AS Roma in 2008 but decided against it due to the club’s debt problems.
Mr Soros was likely attracted to Manchester United because of the team’s profitable media rights deals.
“This could be a play by Soros on the strength of Manchester United’s brand and the English Premier League’s growing media rights,” said Philip Hall, a partner at Inner Circle Sports, an investment bank focused on the sports industry.
“The domestic rights are set to increase 70% for the 2013-2014 season and the international media rights, set to be announced in late October or early November, are also expected to come in at a very robust uplift,” he added.
Mr Soros became prominent in 1992 when he successfully bet on the devaluation of the British pound, netting about $1bn in the process and earning him the moniker: “The man who broke the Bank of England”.