Sinonasal undifferentiated carcinoma (SNUC) is a rare, aggressive disease with ambiguous management and poor prognosis. This study aimed to evaluate the role of radiation therapy (RT) and explore the optimal treatment sequence. Retrospective analysis of survival trends of 410 SNUC patients between 1973 and 2015. The 5-year cancer-specific survival (CSS) rate (45.1%) and overall survival (OS) rates (38.1%) were reported in the 84-month median follow-up. Radiotherapy was a prognosticator for improving CSS (hazard ratio [HR] = 0.425, 95% confidence interval [CI]: 0.299-0.603, p = 0.000) and OS (HR = 0.415, 95% CI: 0.303-0.570, p = 0.000), either with surgery (p = 0.000) or without surgery (p = 0.000). However, in a combined therapy of surgery and RT, preoperative and postoperative RT (5-year OS rates were 47.1% and 45.6%, respectively, p = 0.486) were not significantly different. Radiotherapy plays a key role in improving SNUC survival rates. No significant difference in survival rates was observed in preoperative and postoperative RT treatment.