Posttransplant lymphoproliferative disorder (PTLD) is a well-described complication of solid organ and bone marrow transplants. The most common presentation is intra-abdominal lymphadenopathy or single or multiple intraparenchymal masses involving the liver, spleen, or kidneys. Here we describe the imaging and pathology findings of an unusual case of PTLD appearing as an intramuscular forearm lesion in a pediatric male. The manifestation of PTLD as an isolated upper extremity mass in a pediatric patient has to our knowledge not been described.