Background Reduced skeletal muscle mass may impair outcomes of immune checkpoint inhibitors in recurrent or metastatic head and neck squamous cell carcinoma. We evaluated whether temporal muscle thickness (TMT) on routine head computed tomography was associated with treatment response and survival. Methods We retrospectively analyzed 109 patients treated with nivolumab or pembrolizumab between 2017 and 2023 (89 men [81.7%], 20 women [18.3%]; median age 69 years [range 21–88]). TMT was measured on CT images at the level where the Sylvian fissure was most clearly visualized, perpendicular to the temporalis muscle long axis, on both sides, and the mean of the bilateral values was used. Sex-specific cutoffs were defined by time-dependent receiver operating characteristic analysis for 12-month overall survival (5.53 mm for men; 3.92 mm for women). Associations with objective response and survival were assessed using logistic regression and Cox proportional hazards models. Results Objective response was evaluable in 105 patients (complete response 12, partial response 30, stable disease 18, progressive disease 45), yielding a response rate of 40.0% and a disease control rate of 57.1%. High temporal muscle thickness was associated with response (univariable odds ratio 2.64, 95% confidence interval 1.18–5.90; p = 0.018) and remained significant after adjustment for chemotherapy (adjusted odds ratio 2.42, 95% confidence interval 1.05–5.58; p = 0.038). One-year progression-free survival was 36.5% in the high group versus 15.8% in the low group (absolute difference 20.7%; p = 0.010; adjusted hazard ratio 0.57, 95% confidence interval 0.36–0.90; p = 0.017). One-year overall survival was 73.1% versus 47.4% (absolute difference 25.7%; p = 0.001; adjusted hazard ratio 0.54, 95% confidence interval 0.34–0.86; p = 0.010). Conclusion Temporal muscle thickness, obtainable from routine head computed tomography, was independently associated with response and survival in patients receiving immune checkpoint inhibitors and may serve as a practical host-related imaging marker for prognostic risk stratification.