George Soros, who built one of the world’s largest fortunes through a famous series of trades, has turned over nearly $18 billion to Open Society Foundations, according to foundation officials, a move that transforms both the philanthropy he founded and the investment firm supplying its wealth.

Now holding the bulk of Mr. Soros’s fortune, Open Society has vaulted to the top ranks of philanthropic organizations, appearing to become the second largest in the U.S. by assets after the Bill and Melinda Gates Foundation, based on 2014 figures from the National Philanthropic Trust.

Soros Fund Management LLC’s 87-year-old founder now shares influence over the firm’s strategy with an investment committee of Open Society. Mr. Soros set up the committee and is its chairman, but it is meant to survive him, people familiar with it said.

A new chief investment officer at the Soros firm is less a trader than an allocator of capital to various internal and external asset managers. Unlike past investment chiefs, the official, Dawn Fitzpatrick, doesn’t report to Mr. Soros or others at his firm but to the philanthropy’s investment committee.

Mr. Soros doesn’t plan to trade the billions that now belong to Open Society, according to the people familiar with the situation. Mr. Soros was trading his own money, held separately within the Soros firm, as recently as last year, when he bet—wrongly, it turned out—that stocks would slump after Donald Trump was elected president.