The South Island saddleback is a medium-sized songbird. Adults are black with a chestnut-coloured saddle across their back, while juveniles (commonly referred to as ‘jackbirds’) are entirely covered in chocolate brown plumage for their first year. Saddlebacks have distinctive fleshy, orange wattles on the sides of their mouths. The wattles of fledglings are small and indistinguishable from the gape; older birds generally have larger wattles than younger birds. Sexes are difficult to distinguish in the field except through differences in behaviour (i.e. males courtship feed their female partners, and only females incubate eggs). Males tend to have larger wattles and longer tarsus and wing lengths, and are heavier than females but the sexes can overlap in their measurements.

Voice: vocalisations of the South Island saddleback can be described as cheeet ta-chet ta-chet ta-chet or cheee cheee cheee. Phrases of some vocalisations are often shared between neighbours, which is important in maintaining territories year-round.

Similar species: North Island saddleback (which does not overlap in range) does not have a separate juvenile (jackbird) plumage, has a pale yellow band at the front edge of the saddle, and is slightly smaller with larger wattles, a slimmer bill, and different vocalisations. The female Eurasian blackbird is similar in size and plumage to a jackbird, but is more wary, and has a shorter beak and quite different body shape and posture.