Peritoneal lymphomatosis is an extremely rare presentation of non-Hodgkin lymphoma. We report a case of peritoneal lymphomatosis diagnosed by single-port laparoscopic biopsy. A 70-year-old woman presented to our hospital with a 2-day history of increasing abdominal distension. Abdominal CT and positron emission tomography/CT(PET-CT)demonstrated extensive disseminated disease with marked thickening of the peritoneal surfaces, and a large omental cake with large volume ascites. Under the diagnosis of peritoneal carcinoma, single-port laparoscopic biopsy was performed. Pathological and immunohistochemical examination revealed diffuse large B-cell lymphoma presenting as peritoneal lymphomatosis. She was treated with a combination chemotherapy consisting of rituximab, cyclophosphamide, doxorubicin, and prednisolone(R- CHOP), and no recurrence was reported for 1 year and 6 months. Single-port laparoscopic biopsy was minimally invasive, and helpful for an urgent and accurate diagnosis and treatment of the disseminated peritoneal disease.