Secretariat (March 30, 1970 – October 4, 1989), also known as Big Red, was a champion American thoroughbred racehorse who was the ninth winner of the American Triple Crown, setting and still holding the fastest time record in all three of its constituent races. He became the first Triple Crown winner in 25 years and his record-breaking victory in the Belmont Stakes, which he won by 31 lengths, is often considered the greatest race ever run by a thoroughbred racehorse. During his racing career, he won five Eclipse Awards, including Horse of the Year honors at ages two and three. Widely regarded as one of the greatest racehorses of all time, he was nominated to the National Museum of Racing and Hall of Fame in 1974. In the Blood-Horse magazine List of the Top 100 U.S. Racehorses of the 20th Century, Secretariat was second to Man o' War.

At age two, Secretariat finished fourth in his 1972 debut in a maiden race, but then won seven of his remaining eight starts, including five stakes victories. His only loss during this period was in the Champagne Stakes, where he finished first but was disqualified to second for interference. He received the Eclipse Award for champion two-year-old colt, and also was the 1972 Horse of the Year, a rare honor for a horse so young.

At age three, Secretariat not only won the Triple Crown, but he also set speed records in all three races. His time in the Kentucky Derby still stands as the Churchill Downs track record for 1+1⁄4 miles, and his time in the Belmont Stakes stands as the American record for 1+1⁄2 miles on the dirt. In 2012, his actual time of 1:53 in the Preakness Stakes was recognized as a stakes record after an official review.

Secretariat's win in the Gotham Stakes tied the track record for 1 mile, he set a world record in the Marlboro Cup at 1+1⁄8 miles and further proved his versatility by winning two major stakes races on turf. He lost three times that year: in the Wood Memorial, Whitney, and Woodward Stakes, but the brilliance of his nine wins made him an American icon. He won his second Horse of the Year title, plus Eclipse Awards for champion three-year-old colt and champion turf horse.

At the beginning of his three-year-old year, Secretariat was syndicated for a record-breaking $6.08 million (equivalent to $43.1 million in 2024), on the condition that he be retired from racing by the end of the year. Although he sired several successful racehorses, he ultimately was most influential through his daughters' offspring, becoming the leading broodmare sire in North America in 1992. His daughters produced several notable sires, including Storm Cat, A.P. Indy, Gone West, Dehere, Summer Squall, and Chief's Crown, and through them Secretariat appears in the pedigree of many modern champions. Secretariat died in 1989 as a result of laminitis at age 19.

Secretariat was officially bred by Christopher Chenery's Meadow Stud, but the breeding was actually arranged by Penny Chenery (then known as Penny Tweedy), who had taken over the running of the stable in 1968 when her father became ill. Secretariat was sired by Bold Ruler and his dam was Somethingroyal, a daughter of Princequillo. Bold Ruler was the leading sire in North America from 1963 to 1969 and again in 1973. Owned by the Phipps family, Bold Ruler possessed both speed and stamina, having won the Preakness Stakes and Horse of the Year honors in 1957, and American Champion Sprint Horse honors in 1958. Bold Ruler was retired to stud at Claiborne Farm, but the Phipps family owned most of the mares to which Bold Ruler was bred, and few of his offspring were sold at public auction.

To bring new blood into their breeding program, the Phipps family sometimes negotiated a foal-sharing agreement with other mare owners: Instead of charging a stud fee for Bold Ruler, they would arrange for multiple matings with Bold Ruler, either with two mares in one year or one mare over a two-year period. Assuming two foals were produced, the Phipps family would keep one and the mare's owner would keep the other, with a coin toss determining who received first pick.

Under such an arrangement, Chenery sent two mares to be bred to Bold Ruler in 1968, Hasty Matelda and Somethingroyal. She then sent Cicada and Somethingroyal in 1969. The foal-sharing agreement stated that the winner of the coin toss would get first pick of the foals produced in 1969, while the loser of the toss would get first pick of the foals due in 1970. In the spring of 1969, a colt and filly were produced. In the 1969 breeding season, Cicada did not conceive, leaving only one foal due in the spring of 1970. Thus, the winner of the coin toss would get only one foal (the first pick from 1969), and the loser would get two (the second pick from 1969 and the only foal from 1970). Chenery later said that both owners hoped they would lose the coin toss, which was held in the fall of 1969 in the office of New York Racing Association Chairman Alfred G. Vanderbilt II, with Arthur "Bull" Hancock of Claiborne Farm as witness. Ogden Phipps won the toss and took the 1969 weanling filly out of Somethingroyal. The filly was named The Bride and never won a race, though she did later become a stakes producer. Chenery received the Hasty Matelda colt in 1969 and the as-yet-unborn 1970 foal of Somethingroyal, which turned out to be Secretariat.