Sex hormones play a crucial role in regulating testicular development and maintaining spermatogenesis in the male reproductive system. Knowledge of hormonal regulation in odontocetes is limited to captive species. In this study, the characteristics of hormonal regulation during the testicular development were assessed by histological and immunohistochemical methods in the East Asian finless porpoise (Neophocaena asiaeorientalis sunameri), native to the Chinese Yellow/Bohai Sea coast, China. The testes mass, seminiferous tubule cross section diameter, thickness of the tunica albuginea, and the level of testosterone (T) expression increased abruptly at the age of 3-3.5 years (body length 140-145 cm). However, the estradiol (E-2) expression level decreased with age after 3 years. Therefore, we inferred that the male East Asian finless porpoise (EAFP) > 3 years old (body length > 140 cm) could be classified as the age of puberty onset. Immuno-localization with T was only observed in the interstitial fluid of all animals at all ages. In contrast, a positive reaction for E-2 and its receptor could be observed in the Leydig, myoid, Sertoli, and germ cells at different developmental stages. T is presumed to maintain the tubular microenvironment for spermatogenesis while E-2 may directly regulate spermatogenesis at the level of germ cells. Our findings provide useful information for understanding reproductive status and hormonal regulation in the male EAFP.