A man in his 70s presented to Kyojinkai Komatsu Hospital with lower left abdominal pain and hematochezia after repeated use of laxatives. Computed tomography (CT) revealed continuous bowel wall thickening from the descending colon to the rectum. The symptoms and CT findings were consistent with ischemic colitis. The patient's condition improved with conservative treatment. However, the patient was reexamined 3 months later because complaints of constipation and voiding difficulty continued. Colonoscopic findings revealed rectal stenosis and reddish edematous mucosa with nodular alterations. Although CT showed that the abnormality in the descending and sigmoid colon had resolved, the wall thickening and annular stricture of the rectum persisted. The prostate was irregularly enlarged, encircling and compressing the rectum. Rectal biopsy results did not reveal malignancy. However, moderately to poorly differentiated adenocarcinoma was detected by prostate biopsy. Consequently, the patient was diagnosed with prostate cancer with rectal involvement. The rectal wall thickening and the symptoms improved following hormone therapy. Thus, concomitant prostate cancer invasion should be considered when CT reveals continuous colon wall thickening up to the rectum in a patient suspected of ischemic colitis.