A 12-yr-old girl presented with lordosis and an intraperitoneal mass that revealed a tumor attached to the uterine fundus. The fallopian tubes and ovaries were spared. The mass was completely excised, and a patch of the uterine fundus and the proximal one third of the fallopian tubes were resected. The lesion was composed of bland spindle cells that were positive for sex cord-stromal markers, with particularly strong staining for inhibin and CD56, as well as patchy staining for calretinin, WT1, and steroidogenic factor 1. Thus, the patient was diagnosed with a sex cord-stromal tumor, specifically a fibroma, arising from the uterine corpus. The pathogenesis of this tumor is unclear. An ovarian origin in the context of adherence or a tumor arising from sex cord-stromal ectopic tissues cannot be excluded, but seem unlikely. The tumor might appear as a particular form of uterine tumor resembling an ovarian sex cord tumor. However, this tumor would differ from the presently described classical form of uterine tumor resembling an ovarian sex cord tumor owing to a pure stromal differentiation instead of a pure sex cord differentiation. Finally, because of the low risk for recurrence, long-term follow-up was prescribed for the patient.