Ballymoss (1954–1979) was a British-bred Thoroughbred racehorse trained in Ireland by Vincent O'Brien for whom he became the first top-class horse on the Flat. In a racing career that lasted from 1956 until November 1958, the horse ran seventeen times and won eight races. In 1957, he became the first horse trained in Ireland to win the St Leger Stakes. The following season, he was Europe's leading middle-distance horse, winning the King George VI & Queen Elizabeth Stakes and the Prix de l'Arc de Triomphe.

Ballymoss was a chestnut horse standing just under 16 hands high bred by Richard Ball. He was sired by Mossborough, a good racehorse whose best performance was a second place in the Eclipse Stakes. Mossborough was even better as a stallion, siring winners including Belmont Stakes winner Cavan and Epsom Oaks winner Noblesse. Ballymoss's dam, Indian Call, was well bred but unsuccessful as a racehorse and was sold in 1939 for only 15 guineas.

Ballymoss was sold by his breeder at the Doncaster yearling sales. He was bought for 4,500 guineas by Vincent O'Brien, known, at that time, primarily as a leading trainer of steeplechasers. O'Brien was acting on behalf of American businessman John McShain, who operated Barclay Stable in New Jersey plus a racing operation in Ireland where he maintained a home at Killarney. Ballymoss was ridden to most of his important victories by the Australian jockey Scobie Breasley.

Ballymoss ran four times as a two-year-old with limited success. His only victory came in the Laragh Maiden Plate at Leopardstown.

Racing at age three, Ballymoss began by running unplaced in the seven-furlong Madrid Handicap at the Curragh but then won the Trigo Stakes at Leopardstown over one and a half miles. As a result of his win, he attracted some support in the betting for the Epsom Derby, and his odds for the race shortened from 100/1 to 33/1.

In the build-up to the Derby, Ballymoss incurred a minor injury, leading McShain to cancel his plans to travel to England for the race. At Epsom, Ballymoss exceeded expectations by finishing second of the twenty-two runners behind Crepello. In his next race, he won the Irish Derby at the Curragh. At York in August, Ballymoss started odds-on favourite for the Great Voltigeur Stakes but finished well beaten behind Brioche. Possible explanations for his defeat included an inability to cope with the soft ground and a lack of fitness.

In the St Leger Stakes at Doncaster, Ballymoss started at odds of 8/1 against fifteen opponents. His odds had drifted out from 5/1 when heavy rain softened the ground. He won the race by a length from Court Harwell, with Brioche third. On his final start of the year, Ballymoss ran in the Champion Stakes at Newmarket in October. He made no show in the race and finished unplaced behind the French filly Rose Royale. Following this effort, a plan to run Ballymoss in the Washington, D.C. International was abandoned.

At age four, Ballymoss was beaten on his debut when he finished second to Doutelle in the Ormonde Stakes at Chester. He then won the Coronation Cup at Epsom from the French-trained colt Fric and the Eclipse Stakes at Sandown by six lengths from Restoration and Arctic Explorer before running in Britain's most important weight-for-age race, the King George VI & Queen Elizabeth Stakes at Ascot. He was backed down from 11/4 to 7/4 favourite on the day of the race and took the lead a furlong and a half from the finish, pulling clear to win by three lengths from Almeria.