Rational:Collagenous gastritis (CG) is rarely encountered in clinical practice. Here, we reported a case of CG with iron-deficiency anemia as the main symptom.Patient concerns:The patient was a 26-year-old woman who sought medical advice with a chief complaint of recurrent upper abdominal distention and anemia since the last 3 years.Diagnoses:Gastroscopy at admission showed diffuse nodular mucosa. The pathology showed the formation of a belt hyperplasia of collagen in the superficial mucosa along with the infiltration of inflammatory cells. The subepithelial collagen band was 17.68 to 35.73-μm thick and tested positive for Masson staining, thereby confirming the diagnosis of CG.Interventions:A polysaccharide iron complex capsule was given in a dosage of 0.3 t.i.d., p.o. in combination with an omeprazole capsule (20 mg q.d. p.o).Outcomes:The symptoms (upper abdominal distention and anemia) were ameliorated after 8-week treatment. Blood routine showed that the hemoglobin level rose to 91 g/L.Lessons:It is difficult to diagnose CG. Hence, a comprehensive examination based on clinical manifestations, endoscopic findings, and pathological features is required.