The Lightweight class first appeared in 1920 as a category within the Junior TT, open to motorcycles displacing up to 250 cc. Two years later, in 1922, the Lightweight TT became a standalone five-lap race on the Snaefell Mountain Course. The inaugural winner was motorcycle journalist Geoff S. Davison riding a Levis at an average speed of 49.89 mph (80.29 km/h).
Between 1949 and 1976, the Lightweight TT was part of the FIM Motorcycle Grand Prix World Championship, running on the Snaefell Mountain Course for most of this period except between 1954 and 1959, when it was held on the shorter Clypse Course. When the Isle of Man lost FIM World Championship status after 1976, the Lightweight race was dropped from the schedule, with 250 cc machines subsequently running within the Junior TT.
The Lightweight TT was revived in 1995 for two-stroke machines up to 250 cc and four-stroke machines up to 400 cc. By 1999 it was split into two distinct events: a Lightweight 400 for four-strokes and a Lightweight 250 for two-strokes. Both events were dropped after 2004. Brief returns in 2008 and 2009 saw a lightweight class run on the shorter 4.25-mile Billown Circuit in the south of the Isle of Man rather than the Mountain Course, before being dropped again in 2010 on cost grounds.
The event was reintroduced for the 2012 Isle of Man TT on the Mountain Course with a changed technical specification: water-cooled four-stroke twin-cylinder engines not exceeding 650 cc complying with ACU Standing Regulations. This definition created a class for purpose-built racing twins and production-based parallel-twin machines, distinct from the mainstream Supersport and Superbike categories.
From 2012 onward the Lightweight class ran under this twin-cylinder specification until the 2022 edition, when the race was officially renamed the Supertwin TT.
The lap record for the Lightweight TT (under the 650 cc twin-cylinder formula) is held by Michael Dunlop at 18 minutes and 26.543 seconds, an average speed of 122.750 mph (197.547 km/h), set during the 2018 race. The race record over four laps of the Mountain Course — a total distance of 150.73 miles (242.58 km) — is 1 hour, 15 minutes and 5.032 seconds, an average race speed of 120.601 mph (194.088 km/h), also set by Dunlop during the 2018 event.
The Lightweight TT's long and interrupted history reflects the broader evolution of motorcycle racing's smaller classes. Its repeated revivals underscore the enduring appeal of racing lightweight machines on the Mountain Course, while the rebranding as Supertwin TT in 2022 acknowledged that the class had evolved far from its origins as a category for quarter-litre machines into a race for distinctly characterful twin-cylinder motorcycles. The class has offered a platform for manufacturers and riders operating outside the dominant inline-four formula that defines Supersport and Superbike competition.