We investigated treatment outcomes of radiotherapy for solitary plasmacytoma (SP) and prognostic factors affecting survival. Between 1996 and 2010, a total of 38 patients were treated with radiotherapy for histologically proven plasmacytoma without evidence of multiple myeloma. Among these, 16 and 22 patients had SP originating from extramedullary soft tissue (EMP) and bone, respectively. Thirteen patients received adjuvant chemotherapy, and three patients underwent surgery prior to radiotherapy. At a median follow-up of 50 months (range, 8-142), radiotherapy demonstrated excellent local control (5- and 10-year local control rates, 81%). However, the 10-year multiple myeloma-free survival (MMFS) was 54% and the 10-year overall survival (OS) rates was 35%. Solitary bone plasmacytoma (SBP) more frequently progressed to multiple myeloma (MM) than EMP (10-year MMFS, 0% vs. 71%, p = 0.02). Radiotherapy with doses ≥40 Gy demonstrated better local control (10-year LC, 100% vs. 60%, p = 0.04) in SBP. In the multivariate analysis, elevated β₂-microglobulin was a significantly unfavorable prognostic factor affecting OS (p = 0.03). In conclusion, radiotherapy effectively treated SP without significant toxicity. However, progression to MM presents a challenging problem. Novel therapeutics are needed for patients with unfavorable prognostic factors.