PURPOSE: The importance of nivolumab for recurrent or metastatic (R/M) head and neck squamous cell carcinoma (HNSCC) is rapidly increasing. However, prognostic factors have not been determined for predicting treatment outcome. We aimed to investigate the prognostic factors in R/M HNSCC patients treated with nivolumab. METHODS: This retrospective study included 42 patients with R/M HNSCC who received nivolumab therapy. Correlations of overall survival (OS) with various patient characteristics including age, recurrent/metastatic site, performance status (PS), programmed death-ligand 1 positivity, body mass index, neutrophil-to-lymphocyte ratio, modified Glasgow prognostic score (mGPS), previous cetuximab administration, and immune-related adverse events were investigated. RESULTS: The overall response rate and disease control rate were 16.7% and 45.2%, respectively. Estimated 1-year OS and progression-free survival (PFS) were 56.4% and 24.5%, respectively. Multivariate analysis revealed that PS = 2 (hazard ratio 0.147; 95% CI 0.041-0.527; p = 0.003) and mGPS = 2 (hazard ratio 0.188; 95% CI, 0.057-0.620; p = 0.006) were independent predictors of poor OS. Given that the PS and mGPS were independent prognostic factors, we classified patients into three groups according to PS and mGPS: Group 1, both PS and mGPS were 0 or 1 (n = 30); Group 2, either PS or mGPS was 2 (n = 9); Group 3, both PS and mGPS were 2 (n = 3). The OS curves were significantly stratified among the three groups. CONCLUSION: The combination of PS and mGPS accurately predicted OS after nivolumab therapy. Preventive intervention to maintain general condition without simultaneously exceeding level 2 of PS and mGPS might be important for improving treatment outcomes of nivolumab.