Secondary malignancies are a known, albeit uncommon, complication of radiation for prostate cancer, either in the form of external beam radiotherapy or seed-implant brachytherapy. Of these secondary malignancies, mucinous adenocarcinoma of the prostatic urothelium is an extremely rare clinical entity that has only once been reported in the literature. We report the case of an 80-year-old gentleman who initially underwent low-dose brachytherapy for low-risk prostate cancer 18 years ago. He subsequently developed recurrent gross hematuria and obstructive voiding symptoms. He underwent cystoscopy and transurethral resection of a large tumor from within the prostate. Final pathology of the tumor revealed a mucinous adenocarcinoma. Further immunostaining revealed this is likely to have originated from the prostatic urothelium. Given his age, comorbidities, and no clear data demonstrating that aggressive extirpative surgery provides a clinical benefit, we elected to undergo surveillance. Clinicians should be aware of mucinous adenocarcinoma of the prostatic urethra as an extremely rare, radiation-induced malignancy. Once a diagnosis is made, extirpative surgery is an option for localized disease, although prognosis remains poor.