The Main South Line, sometimes referred to as part of the South Island Main Trunk Railway, is a railway line that runs north and south from Lyttelton in New Zealand through Christchurch and along the east coast of the South Island to Invercargill via Dunedin. It is one of the most important railway lines in New Zealand and was one of the first to be built, with construction commencing in the 1860s. At Christchurch, it connects with the Main North Line to Picton, the other part of the South Island Main Trunk.

The Main North Line between Picton and Christchurch and the Main South Line between Lyttelton and Invercargill, running down the east coast of the South Island of New Zealand, are sometimes together referred to collectively as the South Island Main Trunk Railway (SIMT). Construction of a line running the length of the east coast began in the 1860s and was completed all the way from Picton to Invercargill in 1945; the last sections being on the Main North Line south of Picton. The designation "South Island Main Trunk" originally referred to only that line between Christchurch and Invercargill.