Soft tissue involvement in extramedullary plasmacytoma (EMP) is an exceptionally rare occurrence within the spectrum of plasma cell neoplasms. This case report presents the unique scenario of a patient who developed a soft tissue mass EMP subsequent to receiving radiation therapy for a solitary bone plasmacytoma at a distinct anatomical site. The primary objective of this report is to elucidate the clinical characteristics, diagnostic complexities, and management considerations associated with this uncommon presentation. Through a comprehensive review of existing literature, we aim to provide valuable insights and expertise to healthcare providers involved in the assessment and treatment of similar cases.