Gastrocolic fistula secondary to primary gastric lymphoma is a very rare entity. On admission to outpatient clinics, it may be difficult to diagnose gastrocolic fistula, as its clinical symptoms are nonspecific. A 65-year-old man was presented with weight loss, nausea, vomiting, diarrhea, fatigue, foul-smelling eructation, and upper abdominal pain for the last 2 months. He had also been started antituberculosis drugs 2 months ago because of acid-resistant bacillus (ARB) positivity in sputum in a state hospital. Therefore, symptoms such as nausea and vomiting were attributed to the drugs used for tuberculosis. However, nausea and vomiting continued despite stopping the drugs. Upper endoscopical examination revealed a large crater on the posterior wall of gastric corpus. A large fistulous opening to the transverse colon was also identified during endoscopic examination. An upper gastrointestinal x-ray series demonstrated a fistula between the stomach and the transverse colon. Histopathological examination of the gastric biopsy was determined to be primary gastric diffuse large B-cell-type non-Hodgkin's lymphoma. In conclusion, persistent vomiting may suggest a probable gastrocolic fistula despite nonspecific clinical findings. In the literature, the present case represents the first report of a gastrocolic fistula due to gastric lymphoma in a patient with tuberculosis at its initial presentation.