Polymyositis (PM) is a rare but documented manifestation of chronic graft-versus-host disease (cGvHD) post allogeneic hematopoietic cell transplantation (HCT). We present the case of a 70-year-old male patient who developed severe cGvHD-related PM 3 years after undergoing allogeneic HCT for acute myeloid leukemia. The patient responded to steroids and was maintained long-term with hydroxychloroquine as a steroid-sparing agent. We review the literature concerning the diagnosis and treatment of PM as cGvHD as well as the differentiation of this manifestation from other forms of PM.