Giant multilocular prostatic cystadenoma (GMPC) is a rare benign pelvic mass for which complete surgical resection is an accepted treatment of choice. This report presents the first case of complete resolution of GMPC following a 3-year course of luteinising hormone-releasing hormone agonist alongside external beam radiotherapy for the concurrent treatment of unfavorable intermediate-risk prostate cancer. In addition to illustrating the imaging features of the effect of androgen deprivation therapy (ADT) and radiotherapy on GMPC regression, this case provides evidence for considering ADT as an alternative, noninvasive GMPC treatment option in patients in whom surgical treatment is either contraindicated or can be made less invasive by reducing the size of GMPC prior to its removal.