Priorities USA Action, which cannot coordinate with Clinton's campaign but spends heavily in favor of the Democratic presidential nominee, started September with $41.5 million on hand, according to a Federal Election Commission report filed Tuesday. The fundraising haul for the PAC, which does not face the contribution limits a campaign does, came in the wake of the Democratic National Convention and a polling surge for Clinton.
Soros, the chairman of Soros Fund Management, previously contributed $7 million to Priorities USA Action last year. He also gave Clinton's campaign committee the maximum $2,700 donation last year.
Soros has traditionally donated to Democratic causes and candidates. He previously donated to Clinton's 2008 presidential campaign.
Priorities USA this election has spent heavily on ads supporting Clinton and opposing Republican presidential nominee Donald Trump. The PAC aimed to spend $77 million on TV ads from the last week in August until Election Day in November, NBC News reported last month.
Trump has repeatedly criticized Clinton for receiving big checks from donors, though large GOP donors have mostly spurned the billionaire developer during his run.
Other tops donors to Priorities USA in August include hedge fund manager Donald Sussman, who gave $2 million, and entrepreneur Fred Eychaner, who contributed $3 million.