Immune checkpoint inhibitors (ICIs) are the standard treatment for advanced head and neck squamous cell carcinoma (HNSCC). Programmed death-ligand 1 (PD-L1) is clinically assessed before initiating ICIs; however, there are no established biomarkers for predicting the response to immunotherapy. In this study, inflammatory and nutritional parameters were examined to determine the therapeutic outcomes of ICIs for HNSCC. Sixty-five patients with metastatic or recurrent HNSCC who received programmed death-1 (PD-1) blockade were enrolled. Inflammatory and nutritional indices were correlated with patient outcomes, including the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), systemic immune-inflammation index (SII), and prognostic nutritional index (PNI). Patients aged <70 years were significantly associated with a high NLR, whereas those with a performance status of 2 or 3 were closely related to a high NLR, high SII, and low PNI. Although all patients achieved an objective response rate of 24.6% and a disease control rate of 36.9%, the NLR, PLR, SII, and PNI values were not significantly different between responders and non-responders. Univariate analysis showed that the NLR, PLR, SII, and PNI were significant predictors of progression-free survival (PFS) and overall survival (OS). Multivariate analysis identified PNI as an independent predictor of PFS and OS. PNI, as a nutritional marker, was identified as a significant predictor of outcomes following PD-1 blockade administration in patients with advanced HNSCC, compared to inflammatory markers, such as NLR, PLR, and SII.