Auction house Sotheby's had estimated that the pear-shaped Sun-Drop Diamond would fetch between $11 million and $15 million at auction Tuesday in Geneva.
It was bought by a telephone bidder.
The diamond was sold by New York-based company Cora International, which discovered the jewel in South Africa last year.
Gemologists rate the 110.3 carats diamond as fancy vivid yellow - the highest possible color grading.
The color in yellow diamonds is caused by nitrogen impurities trapped within carbon molecules and hardened over the course of millions of years.
The sales price doesn't include the auctioneer's commission and any taxes the buyer might have to pay.