Histiocytic sarcoma (HS) is an uncommon malignant neoplasm arising from mature histiocytes and most commonly characterized by the immunophenotypic expression of CD68, CD163, or lysozyme. Although rare, HS arising as a second primary malignancy following hematolymphoid neoplasms has been reported. To our knowledge, this is the first reported case of HS occurring as a second primary malignancy in a patient with mycosis fungoides (MF), with the retained immunophenotype markers CD30 and CD4.