Investors were expected to remain cautious until the Fed announces its decision on Thursday.
The Dow Jones rose 140.1 points to 16,739.95, while the S&P 500 added 17.22 points to 1,995.31. The Nasdaq was up 28.72 points at 4,889.24.
Figures released on Wednesday showed consumer prices fell 0.1% in August.
Prices were up 0.2% from a year earlier, indicating that inflationary pressures within the US economy remain weak.
Analysts think the Fed's decision is finely-balanced, as while inflation is weak, the economy is growing and the unemployment rate has fallen to 5.1%.
"You can make a strong case either way for the Fed to begin raising interest rates or waiting," Ryan Sweet, a senior economist at Moody's Analytics, told Reuters.
"The prudent risk management approach would argue for them to hold off, but if the Fed was really data dependent there is a very strong case to raise rates on Thursday."
Among individual stocks, Anheuser-Busch InBev rose 6.8% after it said it had made a takeover approach for rival brewing firm SABMiller.
FedEx shares fell 2.8% after the delivery company cut its full-year earnings forecast.
The company predicted earnings per share of $10.40-$10.90 for the year to 31 May, down from a previous forecast of $10.60-$11.10.

The Glasgow Warriors coach will replace Cotter as Scotland coach next summer.
"I would have thought he would already have been looking to go elsewhere," former Scotland scrum-half Lawson said.
"The risk of potentially losing Gregor before Vern Cotter had done his bit - had he taken them through to the next World Cup - is now nullified."
Townsend served as Scotland's backs and attack coach for three years, and his Glasgow players have provided a large percentage of the squad under Cotter.
"In many ways he's perfectly positioned to step into that role," Lawson told BBC Scotland.
"He's obviously been in and around the Scotland coaching environment before and naturally the two professional clubs work very closely with the national team as it is.
"He's gone in at Glasgow, taken a year or so to build the squad he wants, the culture he wants and the playing style he wants.
"And by the time he steps in, he'll have had four to five years as head coach of Glasgow and that will stand him in incredibly good stead."
Cotter's future beyond the summer of 2017 remains unclear, though Lawson thinks approaches from elsewhere may have convinced him to move on after his contract expires next year.
"Obviously somebody has offered him the chance to go elsewhere," added Lawson, who won 31 caps between 2006 and 2012. "Whether that's back to New Zealand or elsewhere we'll see.
"As much as we're shocked and disappointed [at losing Cotter], we're incredibly fortunate that we have one of the best and most sought-after coaches in the northern hemisphere, in the shape of Gregor Townsend, to come in and pick up as head coach."

The hosts led when Harrison ran onto a long goal kick that had bounced over Cambridge keeper Josh Coulson, and tapped the ball into the net.
Ben Williamson hit the post for Cambridge as they looked to level.
Stevenage sealed the win when Connor Ogilvie's drilled cross found Harrison, who converted from close range.
Harrison helped Stevenage secure promotion to League One in 2011, and is back at the club on loan from Chesterfield.
Stevenage caretaker manager Darren Sarll told BBC Three Counties Radio:
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"It was a good performance. We've come away from that knowing that that's still not our best. There's still so much to do in terms of improving us.
"There's real signs of the old Stevenage coming back. Standing shoulder to shoulder, and we won't be beat. I love that mentality, that's the mentality that I've been brought up on here.
"I thought we could have done better in the first half. To be a little more authoritative with our football, to go after Cambridge a little bit more than we did. There's lots to be pleased with, more so the fact we can beat sides above us."
(Close): US markets closed higher as the Federal Reserve begins its two-day meeting which will discuss whether to raise interest rates.
[NEXT_CONCEPT]
Rory Lawson believes the risk of Gregor Townsend leaving Scotland may have influenced the decision to name him as Vern Cotter's successor.
[NEXT_CONCEPT]
Byron Harrison scored twice on his return to Stevenage, securing their first home win since November.