The male reproductive system of spiders consists of paired, tubular testes and often long and convoluted deferent ducts, which are fused near the genital opening to form an ejaculatory duct. In rare cases and as revealed in previous studies the testes can be partly fused (anteriorly or posteriorly), as reported, for example, for Dysderidae (Dysderoidea). In order to determine whether this pattern is also present in other dysderoids, we studied the genital system of Oonopidae for the first time by means of light microscopy. The following species were included, covering both conventionally recognized subfamilies (Gamasomorphinae and Oonopinae): Neoxyphinus ogloblini Birabén, 1953; Silhouettella loricatula (Roewer, 1942); Opopaea recondita Chickering, 1951; Myrmopopaea sp.; Scaphiella hespera Chamberlin, 1924; Lionneta sp.; Orchestina moaba Chamberlin and Ivie, 1935; Oonops balanus Chickering, 1971, and Stenoonops reductus (Bryant, 1942). In contrast to other spider taxa observed to date, all studied goblin spiders possess an unpaired (completely fused) testis, which seems to be a synapomorphic trait for Oonopidae supporting the monophyly of the family. The deferent ducts of goblin spiders are mostly convoluted and of varying diameter whereas the ejaculatory duct is very small. Moreover, we found a remarkable diversity in the organization of the testis and spermatozoa, which underlines the high potential of the reproductive system for systematic studies on Oonopidae as well as other dysderoids. The putative sister group of Oonopidae is Orsolobidae; since the organization of the male genital system of orsolobids is unknown as well, we included one species (Ascuta media Forster, 1956) in the present study. It possesses the typical genital organization for male spiders represented by paired and unfused tubular testes.