The three-judge panel ruled the bidding process was flawed. It also ruled unanimously that the Legislature should have had a chance to veto the most recent deal.
The 150-acre property was sold to Facilities Management West for $100 million. Bidders who lost out filed a lawsuit, and the sale was put on hold last November.
Assemblyman Jose Solorio, one of the plaintiffs who sued to stop the sale of the fairgrounds, called the decision "a great victory for the residents of Orange County."
"I hope the governor lets us keep our fairgrounds public and does not put the fair out to bid again," said Solorio, D-Anaheim.
Guy Lemmon of FMW said the developer's executives are weighing an appeal.
"We disagree with the appellate court's decision," Lemmon said. "We are weighing our options, and considering an appeal. The sales process was open, fair and in full compliance with the law. The appellate court's decision will have a chilling effect on the governor's plans to sell nonessential assets to address the current fiscal crisis."
State lawmakers approved the sale of the fairgrounds in the summer of 2009 to help pay down the state's deficit. However, since then Orange County elected officials who initially favored the sale turned against it.
City News Service contributed to this report.