Sexual size dimorphism (SSD) and mating system diversity are key factors shaping reproductive evolution, yet their relationships with post-mating sexual selection remain incompletely understood. Here, we use a comparative framework across multiple species of Dolomedes fishing spiders to examine how mating systems, sperm production strategies, and proxies of postcopulatory selection relate to SSD. We quantified testes size and spermatogenesis in three closely related species— D. tenebrosus, D. okefinokensis, and D. scriptus —that differ in mating system and SSD. Dolomedes tenebrosus, a monogynous species with extreme female-biased SSD (eSSD) and male self-sacrifice, showed evidence of termination of spermatogenesis at maturity, indicating a terminal investment strategy. Dolomedes okefinokensis exhibited a similar pattern, suggesting convergent evolution toward monogyny. In contrast, D. scriptus, with moderate SSD and a polygynous system, maintained continuous spermatogenesis, consistent with repeated mating opportunities. Expanding to 13 Dolomedes species, we found a strong positive association between eSSD, monogyny, and spermatogenesis termination, alongside increased spermophor volume in eSSD species. These findings suggest that eSSD is associated with both a terminal male reproductive strategy and intensified investment in sperm transfer structures. Together, our results highlight an integrated evolutionary link between sexual size dimorphism, mating system, and male reproductive investment across Dolomedes spiders.