A 22-year-old man was hospitalized for assessment of thrombocytopenia and fever. Examination showed that he had infectious mononucleosis and moderately severe thrombocytopenia that was asymptomatic. Examination of blood smears revealed that the thrombocytopenia was caused by the clumping of platelets. We made a diagnosis of ethylenediaminetetraacetic acid-dependent pseudothrombocytopenia after excluding other infectious mononucleosis-related mechanisms of thrombocytopenia. When the patient recovered from infectious mononucleosis 2 months later, his thrombocytopenia improved, and no platelet clumping in peripheral blood smears was noted. Ethylenediaminetetraacetic acid-dependent pseudothrombocytopenia should always be considered as a possible cause of reported low platelet counts, even in patients with infectious mononucleosis and splenomegaly.