A 10-year-old female koi subspecies (Cyprinus carpio) was presented with bilateral, asymmetric, sessile masses of the periventiductal tissue. The owner had first observed the masses for approximately 1 year before seeking a veterinary evaluation of the fish. Over the past year, the sessile masses continued to grow and develop in size. An incisional biopsy was performed under general anesthesia to further assess the abnormal tissue structures. Based on the histologic evaluation of the submitted biopsy sample, the growth was determined to be a benign spindle cell neoplasm. The koi also was diagnosed with a concurrent genital tract infection based on cytological examination of the reproductive tract fluid. Following transport to the Veterinary Medical Teaching Hospital for further evaluation, a coelomic ultrasound revealed local infiltration of the tissue to the surrounding tissue. To decrease the risks of recurrence following surgical debulking, chemotherapy was recommended to treat the tissue adjacent to the tumor. Cultures obtained from the genital pore resulted in the isolation of Aeromonas spp. susceptible to ceftiofur, which was elected as a preemptive antibiotic. Surgical debulking of the mass was performed, and bleomycin was administered intralesionally and at the periphery of the surgical site. The koi recovered successfully from anesthesia and was discharged later the same day and prescribed an analgesic regimen of meloxicam. After further histological examination of the submitted tissue samples from the periventiductal growths, the tumor was identified as a leiomyoma. No recurrence of the mass was detected 6 months following the surgical procedure and chemotherapy.