The New York Stock Exchange halted trading for more than three hours because of technical problems that it said were not caused by hacking.
The outage was due to "an internal technical issue" and "was not the result of a cyber breach," NYSE said on Twitter on Wednesday.
"We chose to suspend trading on NYSE to avoid problems arising from our technical issue."
The exchange resumed trade about 3:10pm on Wednesday (local time) after having frozen all equity trading just after 11:30am.
The suspension came on a bruising day for US stocks, with all 30 members of the blue-chip Dow index falling.
The Dow Jones Industrial Average finished down 261.49 points (1.47 percent) to 17,515.42.
The broad-based S&P 500 dropped 34.65 (1.66 percent) to 2,046.69, while the tech-rich Nasdaq Composite Index tumbled 87.70 (1.75 percent) to 4,909.76.
Analysts attributed the rout not to the computer problems but to worries about Greece and China, where stocks have plunged more than 30 percent since mid-June.
"We had the NYSE halted but I don't think this was a factor in this decline," said Peter Cardillo, chief market economist at Rockwell Global Capital. "I think this is due to the acceleration of the turmoil in China."
The outage came shortly after United Airlines grounded planes at US airports for an hour due for what it also called a computer problem, sparking concerns of a coordinated cyber attack on major US institutions.
The Department of Homeland Security says it is investigating both incidents but so far has found no sign of malicious activity.
President Barack Obama had been briefed on the NYSE suspension, said White House spokesman Josh Earnest.
Stocks listed on the NYSE continued to trade on other exchanges, including the Nasdaq and the BATS, the third largest US exchange.