The seven-member jury found that Stewart should not have kept an advance he was paid for the show at the Rio Hotel Casino.
The 60-year-old Stewart, who was not in U.S. District Court when the verdict was reached, had said he was unable to perform because of throat surgery several months earlier.
One of his lawyers, Kerry Garvis Wright, said the rock star will appeal.
Steve Morris, a lawyer for the Rio Hotel Casino and its parent company, Harrah's Entertainment, said he was "delighted and relieved" by the verdict.
Jury foreman Stevan Jorgensen, 56, said the jury believed the argument boiled down to a misunderstanding of the contract between the rock star and the casino.
"We felt it was only fair," he said, "that if Mr. Stewart didn't perform the concert that he should give the money back."