Tumor lysis syndrome (TLS) is an important complication associated with hematological malignancies leading to increased morbidity and mortality. Metastatic calcification due to calcium phosphate crystals precipitated in soft tissues is rarely encountered in TLS. We describe a child with non-Hodgkin lymphoma who had gastric mucosal calcification related to severe hyperphosphatemia due to TLS. Upper-gastrointestinal endoscopic examination performed because of abdominal complaints revealed diffuse mucosal white lesions in mucosa of the gastric antrum and corpus. Pathological examination of the mucosa of the gastric corpus showed marked calcification in the lamina propria. We suggest that calcification in mucosa of the gastric corpus may be seen in patients with TLS. We also suggest that gastric mucosal calcifications should be considered in patients with hematological malignancies from TLS.