Background: The prevalence of obesity is anticipated to increase with nearly 1 in 2 adults having obesity by 2030. Previous meta-analysis and prospective studies showed that both obesity and overweight BMI were associated with worse all-cause mortality, cancer-related mortality, and the incidence of multiple types of cancer. Combined modality therapy, such as chemoradiation, often results in significant morbidity among patients with head and neck cancer (HNC). The association of BMI with treatment response, tumor recurrence, and survival outcomes among HNC patients remains unclear. Methods: Our study included a total of 445 patients with non-metastatic head and neck cancer who underwent curative-intent definitive chemoradiation at the Roswell Park Comprehensive Cancer Center from 01/2005 to 01/2021. We evaluated overall survival (OS), progression-free survival (PFS), locoregional failure (LRF), and distant failure (DF). BMI was stratified by normal weight (18.5-24.9 kg/m²), overweight (25.0-29.9 kg/m²), and obese (30 kg/m² or higher). Evaluation was performed using Kaplan-Meier method, log-rank tests, Cox proportional hazard multivariable (MVA) analyses. Results: A total of 445 patients (373 male [83.8%], median [interquartile range] age, 61 [55-66] years; n = 107 with normal, n = 179 with overweight, and n = 159 with obese) were included for analysis. Median follow up was 48.1 months (interquartile range 24.7-74.9). On Cox MVA, only overweight was associated with improved OS (adjusted hazards ratio [aHR] 0.59, 95% confidence interval [CI] 0.39-0.91, p = 0.02) and PFS (aHR 0.51, 95% CI 0.34-0.75, p < 0.001). On logistic MVA, overweight (adjusted odds ratio [aOR] 0.86, 95% CI 0.80-0.93, p < 0.001) and obese (aOR 0.89, 95% CI 0.81-0.96, p = 0.005) were associated with complete metabolic response on follow-up PET-CT after treatments. On Fine-Gray MVA overweight was associated with reduction in LRF (aHR 0.30, 95% CI 0.12-0.71, p = 0.007), but not DF (aHR 0.92, 95% CI 0.47-1.77, p = 0.79). Being obese was not associated with both LRF (aHR0.63, 95% CI 0.29-1.37, p = 0.24) and DF (aHR 0.70, 95% CI 0.35-1.38, p = 0.30). Conclusions: When compared to normal BMI, overweight BMI was an independent, favorable prognostic factor for complete response after treatments, OS, PFS and a reduction in LRF. Further investigations are warranted to improve our understanding on the role of BMI among patients with HNC.