In this study, we examine the male reproductive apparatus and sperm morphology of Thaumastocoris peregrinus (Thaumastocoridae), a pest of Eucalyptus plantations, to explore its phylogenetic implications using light and transmission electron microscopy. The reproductive apparatus comprises paired testes with three follicles each, dilated efferent ducts acting as seminal vesicles, paired accessory glands, and ejaculatory duct that connects with an aedeagus. Sperm are filiform, around 465 µm long, with short, fusiform nuclei (<
6 µm) and an acrosome observable only under TEM. Key synapomorphies of Heteroptera, including symmetrical mitochondrial derivatives with paracrystalline inclusions and axoneme-derivative bridges, are present. The testicular follicles had mostly late spermatids and no evident differentiation zone, which was different from previously described. These distinctive features may support the recent phylogenetic placement of Thaumastocoridae as a sister group to Pentatomomorpha.