Background: We aimed to evaluate the effect of sarcopenia on survival in head and neck squamous cell carcinoma patients treated with chemoradiotherapy. Materials & methods: Disease-free survival and overall survival were compared according to cervical computed tomography for radiotherapy in 123 sarcopenic and non-sarcopenic patients with locally advanced head and neck squamous cell carcinoma treated with chemoradiotherapy with weekly cisplatin. Results: In multivariate analyses, pretreatment sarcopenia was associated with lower disease-free survival (hazard ratio: 2.60; 95% CI: 1.38–4.87; p = 0.003) and overall survival (hazard ratio: 2.86; 95% CI: 1.40–5.85; p = 0.004). Sarcopenic patients experienced more frequent radiotherapy-related toxicities and platinum-related side effects than non-sarcopenic patients. Conclusion: Sarcopenia could be a potential biomarker to predict prognosis and treatment toxicity in head and neck squamous cell carcinoma.