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Goblin spiders (Oonopidae Simon, 1890) are distributed worldwide and among the most species-rich spider taxa. However, goblin spiders are understudied in many aspects and their phylogenetic relationships are not well resolved. As previously shown for numerous other spider groups the male and female reproductive system bears many characters of phylogenetic relevance. Moreover, the diversity of sperm structures within spiders is astonishingly diverse and often taxon specific. In the present study, we analyzed the primary male reproductive system and spermatozoa of goblin spiders for the first time. We investigated 18 species of 13 genera representing the subfamilies Orchestininae and Oonopinae by means of light and transmission electron microscopy. We scored 44 characters from the gross morphology of the reproductive system as well as spermatozoa including four new characters for the male spider reproductive system. All investigated species transfer sperm as synspermia, a method corroborating with the recently proposed “Synspermiata” clade unifying all ecribellate Haplogynae. Furthermore, goblin spiders show by far the highest diversity of sperm structures in spiders. In total, we recovered 30 unambiguous synapomorphies for different oonopid taxa. In a comparison with all other spider taxa studied to date, we identified the longest sperm (Neoxyphinus termitophilus) and longest sperm conjugates (Orchestina). Moreover and most remarkable is the presence of aflagellate sperm in Opopaea apicalis, which is the first report of the loss of a sperm flagellum in tetrapulmonate arachnids. These findings are of high interest not only because of their phylogenetic implications, but also with regard to their contribution to our understanding of postcopulatory sexual selection in spiders.