Peritoneal lymphomatosis (PL) is a rare extranodal involvement of non-Hodgkin's lymphoma (NHL) and is associated with a poor prognosis. It is confused with the more common peritoneal carcinomatosis and may be misdiagnosed. Early diagnosis is the most important step of effective treatment. In patients with NHL, fluorine-18-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/ CT) is a valuable imaging modality in guiding biopsy, determining extent of the disease, and evaluating metabolic response to therapy. It also provides important information in the differential diagnosis. We report a case of a 40-year-old male patient who presented with abdominal distention and pain, with diffuse thickening of the peritoneum suggestive of peritoneal carcinomatosis on a computed tomography scan. Biopsy from the thickened peritoneum confirmed the diagnosis of diffuse large B cell lymphoma (DLBCL). Fluorine-18-FDG PET/CT performed for staging showed thickening and increased 18F-FDG uptake in almost the entire peritoneum as well as lymph node involvement in supra- and infra-diaphragmatic areas, and a mass in the spleen. Post-treatment 18F-FDG PET/CT revealed a complete metabolic response.