The authors describe a rare tumor with overlapping features of true histiocytic sarcoma and interdigitating dendritic cell sarcoma occurring in infancy. A 3-month-old boy presented with hepatosplenomegaly and abdominal distention. He presented a diagnostic challenge at initial presentation despite extensive investigations. On follow-up at age 16 months he had progressive splenomegaly, thrombocytopenia, and worsening coagulopathy. A diagnosis of histiocytic sarcoma with interdigitating dendritic cell differentiation was established after an open biopsy of liver and spleen. A repeat liver biopsy coupled with splenic biopsy is recommended in young children with unexplained hepatosplenomegaly and thrombocytopenia.