Background: Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the gastrointestinal system. The rectum is a rare location for GIST. Prostate adenocarcinoma is the most common malignancy in geriatric men. Rarely, rectal GIST mimics prostate pathologies. Case Report: We describe a 58-year-old male patient who was admitted with signs and symptoms of prostatism. A presumptive diagnosis of primary prostate sarcoma was made based on imaging studies and a trucut biopsy. Removal of the mass compressing both the prostate and the rectum revealed the final diagnosis of synchronous prostate adenocarcinoma and high-grade GIST originating from the rectum. The patient also had a family history of GIST. Conclusion: According to our knowledge, there are 5 more reported cases of rectal GIST, which were misdiagnosed as prostate malignancy. Rectal GIST may simulate prostate carcinoma clinically, and should always be kept in mind in the differential diagnosis of prostate pathologies.