An 87-year-old woman was referred to our hospital with early rectal cancer and massive ascites. Tuberculous peritonitis was suspected because positron emission tomography-computed tomography showed high uptake in the hypertrophic peritoneum. A staging laparoscopy was performed and the diagnosis of tuberculous peritonitis was established from inspection of histopathological biopsy specimens showing tiny white nodules on the peritoneum, Langhans giant cells, and epithelioid cell granulomas. Tuberculosis bacterium was also detected from this tissue. After 4 months' treatment for tuberculous peritonitis, laparoscopy assisted low-anterior resection was performed. Laparoscopy was used to assess the status of tuberculous peritonitis from before to after treatment, and treatment for rectal cancer was instituted.