According to a statement from the SEC, the new rules require floating net asset values (NAV) for shares of institutional prime money-market funds. Daily share prices will now fluctuate along with the value of the assets held in the fund. Today's reforms also give money-market funds the power to levy fees and "redemption gates" that will discourage massive runs from money-market funds in the event of significant market turmoil, such as what was experienced in 2008.

"Today's reforms fundamentally change the way that money-market funds operate," said SEC Chair Mary Jo White.  "They will reduce the risk of runs in money-market funds and provide important new tools that will help further protect investors and the financial system. Together, this strong reform package will make our markets more resilient and enhance transparency and fairness of these products for America's investors."

Nearly $3 trillion is invested in money-market funds. As of July 3, 2014, more than $800 billion was held in the institutional money-market funds affected by today's reforms, according to the SEC.

"Nobody knows how these assets will be allocated in the wake of today's reforms," Piwowar said in a speech during today's open meeting.

See also: Fed bank leaders object to SEC's money-market plan

During the financial crisis of 2008, institutional money-market funds experienced an unprecedented investor exodus. In one week alone, $300 billion of institutional money was pulled from the funds, or 14 percent of total assets in money markets.

That caused the short-term financing available to corporations, whose bonds are sold through money-market funds, to dry up, posing a severe systemic risk to the U.S. economy. In order to halt the run, the Department of Treasury had to step in to guarantee the investments in money markets.

Today's reforms come in the face of significant resistance from the private sector.

Concerns over tax implications, and the fear that institutional investors like pension funds will be forced to park cash in riskier assets as a result of the reforms, led trade organizations and individual companies to call for a halt in the reforms.

In a statement prior to the vote, Tim Pawlenty, president of the Financial Services Roundtable, said the new rules could "negatively impact American charities, hospitals, universities, nonprofits, and the state and local governments that rely on money-market funds for retirement plans and other cash needs."

Addressing the potential unintentional consequences to the reforms, commissioner Piwowar said that industry's opposition to the reforms suggests that "many institutional money-market fund sponsors and many, if not most, institutional investors could decide to abandon prime money-market funds."

That prospect would greatly impact the short-term funding to state and local governments and businesses, Piwowar said.

See also: Fidelity joins critics of SEC's money market proposals