The Junior TT dates to 1911, the year the TT moved from the St John's Short Course to the full Snaefell Mountain Course. That first Junior race was open to 300 cc single-cylinder and 340 cc twin-cylinder machines, contested over five laps of the new 37.5-mile circuit. Percy J. Evans won the inaugural event on a Humber motorcycle in 3 hours, 37 minutes and 7 seconds at an average speed of 41.45 mph. From 1912, the Junior TT was restricted to 350 cc machines, a capacity limit that remained in effect until 1994 when the class began its transition toward Supersport specifications.
Between 1949 and 1976, the Junior TT was a round of the Grand Prix motorcycle racing World Championship. The removal of the Isle of Man from the FIM calendar after 1976 on safety grounds brought this era to a close. In subsequent decades the class evolved in parallel with FIM Supersport Championship regulations, eventually settling into its current form as a 600 cc Supersport event while retaining the "Junior TT" name in official race schedules before the Supersport designation became more widely used.
Competitors must hold a valid National Entrants or FIM Sponsors Licence for Road Racing. The Supersport TT specification covers four-cylinder four-stroke engines over 400 cc up to 600 cc, three-cylinder four-stroke engines over 600 cc up to 675 cc, and twin-cylinder four-stroke engines over 600 cc up to 750 cc. The official qualification standard requires a lap time within 115 percent of the time set by the third fastest qualifier.
The lap record for the Supersport TT (Junior TT class) is 17 minutes and 31.328 seconds at an average speed of 129.197 mph (207.922 km/h), set by Michael Dunlop on lap 2 of the 2018 Isle of Man TT Supersport Race 1. The race record over four laps is 1 hour, 11 minutes and 28.059 seconds at an average race speed of 126.703 mph (203.909 km/h), set by Dean Harrison during the 2018 Supersport TT Race 2.
Unlike the Senior TT, which runs as a single race on Saturday of race week, the Supersport TT is typically run as two separate races across the week, providing additional track time for the class and increasing its presence on the schedule. This format reflects the popularity of the middleweight Supersport category and allows more racing opportunities for the large number of riders who compete in this class at the TT.
The Supersport TT has produced some of the closest and most competitive racing at the Isle of Man TT, with lighter and more nimble Supersport machines rewarding precise riding and technical skill on the 219-turn Snaefell Mountain Course. The class has served as a proving ground for riders who go on to compete at the Senior TT level, and many of the sport's most celebrated TT competitors have built their reputations through strong performances in the Supersport races.