An 82-year-old Japanese man presented with a gastric involvement of peripheral T-cell lymphoma, not otherwise specified. Although gastrointestinal lesions were not detected on computed tomography, oesophagogastroduodenoscopy revealed a slight elevation of the gastric mucosa, with changes in mucosal colour and the presence of abnormal microvessels. This led to the prompt detection of gastric involvement in lymphoma. This case highlights the usefulness of detailed observation of the gastric mucosa for the endoscopic detection of gastric involvement of peripheral T-cell lymphoma.