Strong winds of up to 60mph are believed to have caused the accident at the Indiana State Fair, in which a further 40 people are thought to have been injured.
The collapse happened at around 9pm local time as some of the 12,000 concertgoers were being evacuated from the Hoosier Lottery Grandstand, according to the Indianapolis Star. Some people were trapped under the wreckage.
An onlooker told the BBC: "It was the worst thing I have witnessed in my life. We could see the sky get really dark and there was lightning and thunder. It almost seemed like a hurricane hit out of nowhere.
"The stage just leaned to the right and crashed down. It felt to me like it was in slow motion. Everyone was scrambling and it was horrifying because you knew people were underneath it."
Stacia Matthews, a reporter for RTV6, described the scene as "pure pandemonium", while Indiana State Police spokesman David Bursten warned that the number of casualties could rise.
Sugarland's singer Jennifer Nettles said the band were "stunned and heartbroken" by the incident.
"We hold those injured in our prayers at this very sad time," she said. "There are no words. It is tragic."
Watch a video of the stage collapse at the Indiana State Fair below:
Warning: contains disturbing scenes