Sinndar (27 February 1997 – 26 November 2018) was an Irish Thoroughbred racehorse and sire. In a racing career that lasted from September 1999 until October 2000, he ran eight times and won seven times. He was notable for his unique achievement in winning the Derby, Irish Derby and Prix de l'Arc de Triomphe in the same year.

Sinndar, a dark-coated bay horse with a white stripe and white hind feet, was bred by his owner the Aga Khan in Ireland. He was sired by Grand Lodge out of the mare Sinntara. Apart from Sinndar, Grand Lodge (winner of the Dewhurst Stakes and the St James's Palace Stakes) sired the winners of over six hundred races including Grandera, Indian Lodge (Prix du Moulin de Longchamp, Prix de la Forêt) and Queens Logic (undefeated two-year-old champion). Sinntara was a staying Listed race winner, descended from a highly successful family which had been established by Marcel Boussac. As a descendant of the broodmare Tourzima, Sinntara was from the same branch of Thoroughbred Family 13-c which produced Darshaan, Acamas (Prix du Jockey Club), Akarad (Grand Prix de Saint-Cloud) and Akiyda (Prix de l'Arc de Triomphe). His pedigree was seen as combining speed on his sire's side with stamina from his dam.

Like many of his owner's horses, he was sent into training with John Oxx near the Curragh, County Kildare, Ireland. Sinndar was ridden in all his races by the stable jockey, Johnny Murtagh.

Sinndar did not appear on the racecourse until the September of his two-year-old season when he ran in a maiden race at his local course, the Curragh. Starting the Evens favourite, he took the lead in the final furlong and won "comfortably" by one and a half lengths from Rainbow And Gold, with the future Melbourne Cup winner Media Puzzle a neck away in third place.

Only two weeks after his debut, having never previously run in a Conditions race, Sinndar was sent to contest Ireland's most important two-year-old race, the National Stakes over the same course and distance. He was regarded as a serious candidate, starting at odds of 7/1, but the overwhelming favourite was the Aidan O'Brien-trained Bernstein, who started at 4/11. Bernstein led from the start, and went clear of his opponents, but suddenly weakened inside the final furlong and was passed by the 50/1 outsider Murawwi. Sinndar, who had been apparently struggling to make an impression, began to stay on strongly, catching first Bernstein and then Murawwi to win the Group One prize by a head. After the race, Oxx characterised the horse as "tough and very honest" whilst the Aga Khan expressed his view that Sinndar had the potential to win more important races over longer distances.

In 2000, Sinndar was clearly aimed at a staying campaign, bypassing the usual Guineas trials and opening his season with a run in the Listed Ballysax Stakes. The race is one in which horses carry extra weight according to races they have already won, making it, unofficially, a form of handicap. As a Group One winner, Sinndar carried seven pounds more than his three opponents, and he was only third in the betting. Johnny Murtagh attempted to make all the running on Sinndar, but the colt was caught inside the final furlong and beaten a head by the Dermot Weld-trained Grand Finale, with the pair finishing fifteen lengths clear of the other two runners. It proved to be Sinndar's only defeat.

A month later, in the Derrinstown Stud Derby Trial, Sinndar again gave seven pounds to the other runners, who included his stable companion Shawandi, who was in the race to set the pace, and the unbeaten Bach from Aidan O'Brien's Ballydoyle stable. Sinndar and Bach were made joint favourites and dominated the finish after Shawandi gave up the lead in the straight. Both colts ran on strongly under pressure, with Sinndar winning by a head.

Sinndar's next race was the Derby, although until just before the race, the possibility of firm ground at Epsom had led Oxx to consider running the colt in the Prix du Jockey Club- the French Derby instead. Despite the late withdrawal of the 2,000 Guineas winner King's Best, the field for the 2000 Derby was a strong one, and Sinndar was fourth in the betting behind Sakhee, Beat Hollow and the Ballydoyle-trained Racing Post Trophy winner Aristotle, with Best of the Bests, representing Godolphin also well supported. Oxx was confident of a prominent showing, saying, "Our fellow will run well and I'll be disappointed if he doesn't run a cracking race and be involved in the finish".