Steve Wagner, a spokesman for the Dallas Safari Club, which sponsored the closed-door event, confirmed the sale of the permit for a hunt in Namibia. He declined to name the buyer.
Ben Carter, executive director of the Safari Club, has defended the auction, saying all money raised will go towards protecting the species.
But the auction drew howls from critics, including wildlife and animal rights groups, and the FBI earlier this week said it was investigating death threats against members of the club.
About 40 protesters gathered outside the convention centre where the auction and a pre-auction dinner took place.
They held signs and chanted.