Quadrangle (April 16, 1961 – September 28, 1978) was an American Thoroughbred racehorse who won the 1964 Belmont Stakes.
He was out of the mare Tap Day, a daughter of the Calumet Farm champion stallion Bull Lea.
Triple Crown when he won the Belmont Stakes, which was run that year at Aqueduct Racetrack.
In 1964, Quadrangle also faced several other top-quality three-year-olds including Hill Rise and Roman Brother, plus older horses such as Kelso and Gun Bow.
Retired to stud, at the Blueridge Farm near Upperville, Quadrangle was a successful sire of a number of stakes winners plus a Hall of Fame filly and Eclipse Award winners, including: While still at stud, Quadrangle broke a leg and was humanely destroyed in 1978 at age seventeen.