The 40-year-old artist was one of the headliners at the Route 91 Harvest Festival and was performing when gunshots were fired.
In a video posted on social media Aldean and his band stop playing, apparently during the last song of his set, after what sounds like multiple gunshots ring out and screams are heard.
According to the festival billing, he was the last person scheduled to perform at the Sunday night concert, beginning at 9.40 local time.
A representative confirmed to The Independent that Aldean and his team are safe. Fellow artist Luke Combs, who performed shortly before Aldean, is also OK.
Luke Owen also tweeted to confirm he was safe, writing that he witnessed the most "unimaginable event".
Video footage showed people screaming and running as multiple gunshots were heard. Police have said "one suspect is down".
Speaking to NBC Las Vegas, two festival-goers said they heard a noise "like firecrackers" but it quickly became apparent that a shooter or shooters were unloading "clip after clip" into the crowd.
At least 14 of those taken to hospital after the shooting are critically wounded, a hospital spokesman said.
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Another hospital spokesperson told Reuters that at least 20 people have been killed and dozens more injured.
This was the fourth time the Route 91 festival was held in Las Vegas. Among the performers were some of America's biggest country artists, including Eric Church, Sam Hunt, Owen and Aldean.
For the latest updates on Las Vegas, follow our liveblog here. The Independent has reached out to Aldean's team for comment.
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