Birdstone (foaled May 16, 2001, in Kentucky) is an American Thoroughbred racehorse best known for winning the 2004 Belmont Stakes and has become a successful sire.

On August 28, 2020 Birdstone was pensioned from stud duty to Old Friends Retirement Farm in Georgetown, Kentucky where he will be accessible to his fans from all over the world.

He was not a factor in the first two Triple Crown races (finishing eighth in the Kentucky Derby and skipping the Preakness Stakes) and was a 36-1 longshot when he upset the overwhelmingly favored Smarty Jones, taking the lead in the final furlong (201 m) in the fastest Belmont Stakes since the advent of modern electronic timing (2002 was the first year times were kept to hundredths).

In the winner's circle after the Belmont Stakes, his owners apologized to the connections of Smarty Jones for winning, as did jockey Edgar Prado.

Birdstone was one of the top ten ranked freshman sires of 2008 with 11 winners out of 22 starters and stood for $10,000 for the remainder of the 2009 season, despite his first-crop success.