A 60-year-old woman underwent upper gastrointestinal endoscopy for an abnormality identified during routine examination. The lower gastric corpus showed a type 0-I elevated lesion with a faded mucosa and an area of converging mucosal folds in contact with the lesion. Biopsy indicated the former to be a high-grade adenoma and the latter to be a mucosa-associated lymphoid tissue (MALT) lymphoma. At the same time, Helicobacter pylori infection was diagnosed. Eradication therapy was administered to manage the MALT lymphoma; this resulted in improvement after 3 months. Endoscopic submucosal dissection was performed for the elevated lesion, and subsequent histopathology showed contact between the MALT lymphoma and gastric cancer. Therefore, the patient was diagnosed with a collision tumor. Concurrent cancers are increasingly reported and should be considered during examination.