In　the　coastal　environment,　the　co-occurrence　of　antibiotic　and　nanoplastic　pollution　is　common.　Investigating　their　individual　and　combined　toxicity　to　marine　organisms　is　of　great　necessity.　In　the　present　study,　the　reproductive　toxicity　of　sulfamethazine　(SMZ)　and　nanoplastics　(polystyrene,　PS)　via　the　dietary　route　on　the　spermatogenesis　of　marine　medaka　(Oryzias　melastigma)　was　examined.　After　30　d　of　dietary　exposure,　SMZ　alone　decreased　the　gonadosomatic　index　(GSI)　value　(∼35%)　and　the　proportion　of　undifferentiated　type　A　spermatogonia　(Aund)　(∼40%),　probably　by　disrupting　the　testicular　sex　hormone　production,　the　spermatogenesis-related　growth　factor　network　and　the　balance　of　apoptosis.　Individual　exposure　to　PS　did　not　affect　the　GSI　value　or　the　proportions　of　germ　cells　at　different　developmental　stages,　but　dysregulated　the　expression　of　several　spermatogenesis-related　genes.　Interestingly,　the　presence　of　PS　alleviated　the　decreased　GSI　value　caused　by　SMZ.　This　alleviation　effect　was　achieved　by　enhancing　the　spermatogonia　differentiation　instead　of　reversing　the　suppressed　self-renewal　of　Aund,　suggesting　that　the　mixture　of　PS　and　SMZ　could　cause　reproductive　effects　in　a　different　way.　These　findings　expand　our　knowledge　of　threats　of　ubiquitous　antibiotic　and　nanoplastic　pollution　to　fish　reproduction　and　population.　©　Chinese　Society　for　Oceanography　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature　2024.