Knowledge about reproductive parameters in male harbour porpoises such as testicular histology and germ cell maturation as well as seasonal changes in spermatogenesis is scarce. Thus, the aim of the present study was to report changes in the histological appearance of the testicular morphology of neonatal and juvenile harbour porpoises during maturation, to identify stages of spermatogenesis in adult males and to detect seasonal modifications. The identification of these stages can be used to assess the developmental profile of gene expression during spermatogenesis and to identify defects in spermatogenesis arising in pathological conditions. Testes of adult male harbour porpoises from the North and Baltic Sea that became stranded or by-caught in the years 1998–2016 were histologically examined using Haematoxylin and Eosin – staining. The Periodic Acid Schiff (PAS) staining was used for spermatogenic staging and the evaluation of the development of the acrosomic cap. For the identification of changes in testes morphology and morphometry during the course of the year, histological characteristics like germ cell associations and diameter of the convoluted seminiferous tubules were noted for each month. The analysis showed that in adult males more than one stage of spermatogenesis could be found per cross section of the convoluted seminiferous tubules similar to findings in men and some ape species. This rare phenomenon is called multi-stage-arrangement. In sexually active males from the peak breeding season (June and July) eight stages of spermatogenesis were identified and all stages occurred simultaneously, while during the low breeding season (August to May) only residual spermatogenesis or constituent germ cell populations were found. Missing germ cell generations were recorded in specimens from July to September. Our investigations provide a detailed staging of spermatogenesis and give new insight into the reproductive biology of male harbour porpoises. With these new basic parameters, indicators for endocrine disruptors can be developed in the future, aiming to detect how environmental factors could affect male fertility in wildlife.