The Dow Jones Industrial Average was up 35.69 points (0.28 percent) at 12,631.06 points at about 1640 GMT, minutes after the central bank's policy-setting Federal Open Market Committee (FOMC) announced its decision.
The tech-rich Nasdaq Composite rose 2.71 points (0.10 percent) to 2,850.25 and the S&P 500 edged up 0.75 point (0.06 percent) to 1,347.99.
The FOMC said in a statement it would keep its key interest rate at 0-0.25 percent "for an extended period," a position widely expected after it met Wednesday morning.
The panel said it "will complete purchases of $600 billion of longer-term Treasury securities by the end of the current quarter."
The bond market was essentially unchanged.
Ahead of the keenly anticipated Fed announcement, the main stock indices were little-changed from a day earlier.