South Otago lies in the south east of the South Island of New Zealand. As the name suggests, it forms the southernmost part of the geographical region of Otago.
The exact definition of the area designated as South Otago is imprecise, as the area is defined not in geopolitical or administrative terms, but rather by the area's topographical features and the similarity of its communities. Overall, it encompasses some 8,000 km² (3,100 sq mi) and has a population of approximately 20,000, but these figures vary according to the various definitions of South Otago's boundaries. The area is often seen as roughly congruous with the Clutha District, which has its administrative centre at Balclutha. However, most of the Taieri catchment, from Taieri Mouth and Henley to Mosgiel and Middlemarch, with the coastal areas around Brighton, has been part of the City of Dunedin since 1989. The southwestern part of Clutha District, around the townships of Tapanui and Clinton, are regarded as part of a separate area, West Otago.