Rituximab is a highly effective agent that is used in the treatment of B-cell lymphoma. Rituximab-induced acute thrombocytopenia is a rare side effect that has previously been reported in a small number of patients with malignant lymphoma; its mechanism is still unknown. We herein report the case of a 74-year old man who was diagnosed with follicular lymphoma and who developed severe acute thrombocytopenia the day after the administration of rituximab. Coagulation abnormality, which mimicked disseminated intravascular coagulation, also appeared. When physicians use rituximab to treat high-risk patients, the platelet count should be closely monitored to avoid possible adverse events.