Rhinosinusitis is common in patients with nasopharyngeal carcinoma (NPC). Our study aimed to explore the role of rhinosinusitis severity in NPC prognosis. Medical records and radiologic examinations of 90 patients with NPC at a single medical center from 2009–2016 were retrospectively analyzed. The Lund–Mackay (L–M) score was obtained for each patient before and after 6 months of treatment. Rhinosinusitis diagnosis was based on L–M scores of ≥4. L–M score differences were calculated as pre-treatment rhinosinusitis (PRRS) minus post-treatment rhinosinusitis (PSRS). L–M score difference was sub-grouped into “L–M scores > 0”, “L–M scores = 0”, and “L–M scores < 0”. Clinical staging of our patients based on the American Joint Committee on Cancer 7th edition were: stage I in nine, stage II in seventeen, stage III in twenty-two, and stage IV in forty-two patients; twenty-seven (30%) patients had died. PRRS incidence was 34.4%, and PSRS was 36.7%. Median of L–M scores difference was 0 (−2.2). L–M score difference was an independent prognostic factor for the overall survival of patients with NPC (p < 0.05). Therefore, worsening rhinosinusitis was a prognostic factor for patients with NPC. Clinicians should consider NPC as a warning sign of poor prognosis during routine follow-ups.