Importance: The American Joint Committee on Cancer staging system (Cancer Staging Manual, 8th Edition) for head and neck squamous cell carcinoma (HNSCC) now categorizes human papillomavirus (HPV)-negative HNSCC in a single positive lymph node smaller than 3 cm with pathologic extranodal extension (ENE) as N2a. The standard of care for pathologic ENE is adjuvant chemoradiation therapy (CRT). Whether adding chemotherapy concurrent with adjuvant radiation therapy improves survival in this clinical scenario is unknown. Objective: To assess whether adjuvant CRT relative to radiation therapy alone is associated with improved survival among patients with pN2a HPV-negative HNSCC with ENE. Design, Setting, and Participants: This retrospective cohort study included 504 patients with pN2a HPV-negative HNSCC with ENE who had undergone margin-negative surgery and adjuvant therapy. The patients were identified from the National Cancer Database from January 1, 2004, to December 31, 2015. Statistical analyses were conducted from September 1, 2019, to April 16, 2020. Main Outcomes and Measures: The primary end point was overall survival. The association of adjuvant CRT with overall survival was analyzed using univariate and multivariate Cox proportional hazards regression analyses. Planned subset analyses were conducted in patients younger than 70 years with no comorbidities (the subset most likely to be eligible for a clinical trial of cisplatin-based chemoradiation) and in patients with pT3/T4 disease classification. Results: Of 504 patients (mean [SD] age, 60.5 [12.7] years; 319 [63.3%] men; 434 [86.1%] White) with pN2a HPV-negative HNSCC with ENE who had undergone margin-negative surgery and adjuvant therapy, 298 patients (59.1%) received adjuvant CRT. For the overall cohort of patients with pN2a ENE, adjuvant CRT was not associated with improved overall survival relative to adjuvant radiation therapy alone in a multivariate analysis (adjusted hazard ratio, 0.98; 95% CI, 0.74-1.30). Adjuvant CRT was still not associated with improved overall survival in a subset analysis of 304 patients younger than 70 years with no comorbidities (adjusted hazard ratio, 0.98; 95% CI, 0.66-1.45) nor in a subset of 220 patients with pT3/T4 disease classification (adjusted hazard ratio, 1.03; 95% CI, 0.70-1.54). Conclusions and Relevance: This study found that for patients with pN2a HPV-negative HNSCC with ENE who underwent margin-negative surgery and adjuvant therapy, adding chemotherapy concurrent with adjuvant radiation therapy was not associated with improved overall survival. Additional research is necessary to identify the optimal treatment paradigm for this clinical scenario..