Although thrombotic thrombocytopenic purpura (TTP) is a rare disease, when it develops in a post-cardiac surgery patient, it may have a fatal outcome. Since the frequency of early-onset thrombocytopenia in post-cardiac surgery patients is high, platelet concentrates are commonly transfused during postoperative management. However, when TTP is the likely diagnosis, platelet transfusion is not recommended. We experienced a postoperative TTP in a cardiac surgery patient and discovered the importance of identifying the etiology of postoperative thrombocytopenia. Here, we report the case with a brief review of the literature.