A mucinous cystadenoma of the paratestis arising from an unequivocal oviduct-like structure in an 18-year-old man is reported. Mucinous cystadenomas of the testis and paratestis are extremely rare benign tumors of controversial origin. The diagnosis may be challenging and is of clinical importance because these lesions may mimic a teratoma, and teratomas in postpubertal males carry a malignant potential regardless of the degree of maturation. A few case reports and reviews of testicular and paratesticular mucinous neoplasms resembling ovarian tumors have suggested the possibility of a müllerian origin of these tumors. However, no histologic evidence has been found to support such a histogenesis. The current case demonstrates a clear transition from müllerian-type tubal epithelium to intestinal-type mucinous epithelium. This finding, in conjunction with immunophenotypic studies, strongly supports a derivation from a paratesticular müllerian remnant. To our knowledge this represents the first substantial evidence that, in at least some cases, mucinous cystadenomas of the testis and paratestis are of müllerian derivation.