Phlegmonous gastritis is a rare acute bacterial infection of the gastric wall with an extremely high mortality rate. Early diagnosis is crucial for immediate treatment that could improve the outcomes. Here we report a case in which a patient with underlying chronic myelomonocytic leukemia was diagnosed with phlegmonous gastritis on biopsy. This 57-year-old man presented with shortness of breath and intermittent upper quadrant abdominal pain for 4 days. Laboratory tests showed markedly increased white blood cell. A diagnosis of chronic myelomonocytic leukemia was made based on a peripheral blood smear and flow cytometry. Gastric biopsy showed suppurative inflammation in the submucosal region, prompting the diagnosis of phlegmonous gastritis. The patient was given empirical antibiotic treatment, and the white blood cell decreased dramatically. Surgical intervention was discussed but deferred. Despite continued antibiotics treatment, the patient died. The limited autopsy confirmed the diagnosis of phlegmonous gastritis. Immunohistochemical studies further revealed the occurrence of myeloid sarcoma that involved the gastrointestinal tract.