Unclassified sex cord/gonadal stromal tumors (SCSTs) of the testis comprised predominantly of spindle cells are extremely rare. It is important to differentiate these tumors from other testicular neoplasms, especially other types of sex cord/gonadal stromal tumors. In addition to histological criteria, immunohistochemistry plays an important role in the differential diagnosis. We describe a single case of this rare tumor type and discuss the differential diagnostics options based on a review of the related literature. The problem of the lack of reliable morphological prognostic factors is also addressed.