Background <p>Supernumerary testis, or polyorchidism, is a rare congenital anomaly characterized by the presence of an extra testis anywhere in the line of anatomical descent. Adenomatoid tumors are benign neoplasms of mesothelial origin that can be found anywhere in the genital tract; however, the occurrence of this rare tumor in a congenitally present extra testis is an extremely unique finding to add to the medical literature.</p> Case presentation <p>A 35-year-old male with an unremarkable medical history presented with chronic testicular pain for 2&#xa0;months. On examination, mild, tender, palpable nodular mass was felt in the right scrotal sac. On ultrasonography, a hypoechoic area was identified in the third extra right testis, and on MRI, a small hypointense area is appreciated in the separate right testis on T2-weighted images, and enhancement did not increase on T1-weighted images. The tumor markers for the testicular mass were within normal range, and he was scheduled for surgical exploration. The removed testis was sent to the histopathology section, which revealed microscopically, hematoxylin and eosin-stained sections of the tumor display tubules and cords of cuboidal to columnar cells having moderate eosinophilic cytoplasm with prominent vacuoles, with occasional signet ring morphology, separated by intervening delicate fibrovascular stroma. No hemorrhage or necrosis is noted.</p> Conclusion <p>Adenomatoid tumors are the most common paratesticular tumors; however, their presence in the testis is considered rare, and their presence in the supernumerary testis is an extremely rare occurrence. This case highlights the importance of considering adenomatoid tumors in the differential diagnosis of testicular masses, especially in rare anatomical variations such as supernumerary testes. Our literature review provides a deeper insight into the understanding of this rare entity.</p>