OBJECTIVE: Effective implementation of timely postoperative radiation therapy (PORT) as a quality metric for head and neck squamous cell carcinoma (HNSCC) care may reduce survival disparities in subgroups with the highest risk of mortality-advanced stage and Black race. We sought to determine if the association between timely PORT and survival varies among clinical and demographic high-risk subgroups. METHODS: In this retrospective cohort study using the National Cancer Database 2004-2020, patients with HPV-negative HNSCC treated with surgery and PORT were identified. The association between PORT delay greater than 6 weeks with overall survival was assessed by Cox proportional hazards models with interaction effects for race, overall stage, and PORT delay. RESULTS: Among 54,035 patients included in the study, 62% (33491) had PORT delays. Black patients were more likely to have PORT delays (aOR 1.19, 95% CI: 1.10-1.27). Timely PORT was only associated with improved survival for all stage I patients and White stage IV patients (predicted mortality risk 2.53, 95% CI: 2.34-2.74 versus 2.88, 95% CI: 2.67-3.09, referenced to White, stage I). After adjusting for covariates, timely PORT was estimated to be associated with inferior overall survival in Black patients with stage IV disease although with low precision (predicted mortality risk 3.24, 95% CI: 2.87-3.66, versus 2.71, 95% CI: 2.47-2.98, referenced to White, stage I). CONCLUSION: Timely, guideline-adherent PORT may not reduce survival disparities in Black patients, especially those with advanced stage disease, suggesting other interventions are needed to address this disparity.