Age and sexual maturity have not previously been studied in Australian bottlenose dolphins. These were investigated in 57 male Indo-Pacific bottlenose dolphins (Tursiops aduncus) collected opportunistically in South Australia between 1989 and 2011. Age was estimated by counting growth layer groups in dentine tissue. Formalin-fixed testes plus epididymides were weighed, measured, and subsampled for histological analysis. Stage of spermatogenesis, presence of spermatozoa, testis tubule size, seminiferous tubule wall and tunica thickness, and relative amount of interstitial tissue were noted. Immature testis tubules contained only Sertoli cells and spermatogonia and had mean testis weights < 83 g, lengths < 190 mm, and tubule diameters < 98 μm. Testes of Active dolphins had spermatozoa, testis plus epididymis weights between 250 and 1,025 g, testis lengths between 164 and 292 mm, and mean tubule diameters > 84 μm. Regressed/pathological dolphins had testis weights (199–280 g), testis lengths (195–230 mm), and tubule diameters (59–130 μm) larger than Immature males and on average less than Active males. Seminiferous tubule wall thickness also separated Regressed/pathological male (X̄ = 14.9 μm) from Immature male (X̄ = 2.3 μm) testes. Sexual maturity in male Indo-Pacific bottlenose dolphins was attained between body lengths of 208 and 220 cm and body weights of 90 and 115 kg. Some males (20%) were sexually but not physically mature as determined by fusion of vertebral epiphyses. All physically mature males were sexually mature. The youngest sexually mature dolphin was 12 years old and the oldest immature male was 15 years, but samples were limited in the 8- to 12-year range. Testicular regression has not previously been reported in Tursiops spp. The 3 regressed males from South Australia had gross and histological evidence of organ pathology, including in the testes, suggesting that their reproductive status was related to ill health, rather than seasonal or individual cycling of function or senescence. The implications of heavy-metal toxicity on male Indo-Pacific bottlenose dolphin reproduction is discussed.