The Billown Circuit is a motorcycle road-race course used for the Southern 100, the Pre-TT Classic races and the National Road Races meetings near Castletown on the Isle of Man.

Following the closure of the former RAF Station at Andreas to motorcycle racing in 1954, and to celebrate the success of George 'Sparrow' Costain and Derek Ennett at the 1954 Manx Grand Prix, the Southern Motor Cycle Club (SMCC) proposed that a motorcycle circuit be developed in the area around Castletown in the Isle of Man.

[1] The proposal for a "South TT"[2] using roads around the new Castletown A5 by-pass and the Billown Mansion met with a lukewarm response, and an application for a grant of £500 for race expenditure was rejected by the Tynwald Race Committee in April 1955.

[3] A grant of £500 from local businesses, including T. H. Coleburn for a public address system, enabled the first Southern 100 road race event to be scheduled for July 1955 as a club-level race meeting.

The lap record for the Billown Circuit is 2 minutes 12.735 seconds at an average speed of 115.267 mph (185.504 km/h) set by Michael Dunlop during the 2017 races.