Background: For definitive chemoradiotherapy (chemoRT) of head and neck squamous cell carcinoma (HNSCC), cisplatin is the preferred concurrent agent, with superiority over cetuximab for HPV-associated oropharyngeal squamous carcinoma recently shown in 2 randomized trials (RTOG 1016 and De-ESCALaTE). Patients who are not candidates for cisplatin may be treated with carboplatin instead, but its comparative efficacy is unclear. We analyzed nationwide patterns of care and cancer-specific outcomes after cisplatin- versus carboplatin-based chemoRT. Patients and Methods: Patients with locoregionally advanced (stages III–IVB according to the 6th and 7th editions of the AJCC Cancer Staging Manual) squamous cell carcinoma of the oropharynx, larynx, or hypopharynx who received definitive radiotherapy (RT) were identified in the linked SEER-Medicare database. The concurrent chemotherapy regimen was determined through corresponding Medicare claims. Death caused by HNSCC (cancer-specific mortality [CSM]) was analyzed with competing risks. Propensity score analysis and multivariable Fine-Gray regression were used to adjust for baseline differences, including age and comorbidity. Results: We identified 807 patients who received cisplatin-based chemoRT and 342 who received carboplatin-based chemoRT. Most carboplatin recipients (68%) had combination chemotherapy, predominantly with paclitaxel. Carboplatin- and cisplatin-based chemoRT had similar incidences of death attributable to HNSCC (3-year CSM, 29% vs 26%; P=.19), which persisted in propensity score–matched analysis. In addition, no significant difference in overall survival was seen in the matched cohorts. ChemoRT with either cisplatin or carboplatin was superior to RT alone and RT with concurrent cetuximab. In the multivariable model, the adjusted hazard ratio of CSM for carboplatin relative to cisplatin was 1.01 (95% CI, 0.79–1.28; P=.94). Conclusions: Definitive carboplatin-based chemoRT was equivalent to cisplatin-based therapy and superior to RT alone and RT with concurrent cetuximab. In light of recent results of the RTOG 1016 and De-ESCALaTE trials, our findings suggest that carboplatin-based regimens warrant prospective investigation as an alternative to cisplatin for patients who are not cisplatin candidates.