A 38-year-old woman with pancytopenia and liver dysfunction was diagnosed with active systemic lupus erythematosus (SLE). On days 9 and 10 of admission, peripheral blood smears showed macrophages phagocytosing platelets, and reactive hemophagocytic syndrome (HPS) was diagnosed. Hemophagocytic syndrome was successfully treated with high-dose prednisolone therapy and one course of methylprednisolone pulse therapy. Detection of hemophagocytosing macrophages in peripheral blood smears would be a useful and noninvasive method of diagnosing SLE-associated HPS.