In a global financial environment dominated by negative interest rates and central banks signaling even more accommodative policies, the U.S. money-market industry is thriving.

Normally seen as a place to park cash during times of uncertainty, taxable funds have seen roughly $136 billion of inflows this year, even with U.S. equity markets surging and bonds posting positive returns, Investment Company Institute data show. Overall assets have swelled to more than $3 trillion, the highest level since the financial crisis.

Demand is being aided in part by attractive U.S. short-term yields relative to bank deposits—helped by three years of Federal Reserve interest-rate hikes, an inverted yield curve, and volatility in financial markets. Total assets in government money funds are at a record high, and investments in prime funds are the most since September 2016, before industry reforms went into effect.