Abstract A 4-year-old, spayed female ferret ( Mustela putorius furo) presented for alopecia, pruritus, and swollen vulva. The ferret was treated for a suspected ovarian remnant at 1.5 years of age. On presentation, the physical examination revealed a large, hard, nonpainful caudal intraabdominal mass. The results of diagnostic blood tests and radiographic images were nonspecific relative to determining a definitive diagnosis. An exploratory laparotomy was performed to remove the mass. Histopathology identified the mass as a thecoma. Although neoplasia is common in ferrets, reproductive system malignancies are underreported in this species because most are acquired as gonadectomized pets. The following article describes a case of a sex hormone–producing thecoma in a female ferret with alopecia and swollen vulva.