Rituximab is a chimeric monoclonal antibody directed against the phosphoprotein CD20. Because of its efficacy and acceptable toxicity profile, rituximab is now commonly used for the treatment of CD20-positive B-cell malignancies, including B-cell non-Hodgkin’s lymphoma. However, rituximab-induced acute thrombocytopenia is an extremely rare side effect. We report a case of acute thrombocytopenia occurring immediately after rituximab infusion in a mantle cell lymphoma patient with bone marrow involvement and massive splenomegaly. Although the mechanism of thrombocytopenia is still unclear, it is possible that tumor burden, bone marrow involvement, the presence of infusion-related symptoms, and mantle cell histology are related to this rare complication of rituximab therapy. Hence, rituximab should be used with caution in patients who have these factors, and clinicians must be aware of this rare, but serious, side effect.