Background: Chemotherapy with or without pelvic radiotherapy (RT) is included in the NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) for metastatic anal cancer (MAC), despite limited clinical evidence for RT in this setting. In addition, increasing evidence shows that local therapies, including RT, may increase patient survival for some types of metastatic cancers. The purpose of this study was to evaluate the patterns of care and association between definitive pelvic RT and overall survival (OS) for patients with MAC. Methods: The National Cancer Database was analyzed to evaluate OS of patients with newly diagnosed MAC treated with chemotherapy with or without pelvic RT. Those who did not undergo treatment, treated with surgery, or without baseline variables were excluded. OS was analyzed using the Kaplan-Meier method, log-rank test, Cox proportional hazards models, and propensity score–matched analyses. Results: From 2004 through 2015, 437 patients received chemotherapy alone and 1,020 received pelvic chemoradiotherapy (CRT). At a median follow-up of 17.3 months, CRT was associated with improved OS on univariate (P<.001) and multivariate analysis (hazard ratio [HR], 0.70; 95% CI, 0.61–0.81; P<.001). Propensity score–matched analysis demonstrated superior median survival (21.3 vs 15.9 months) and 2-year OS rates (46% vs 34%) with CRT compared with chemotherapy alone (P<.001). Landmark analyses limited to long-term survivors of ≥1, ≥2, and ≥4 years showed improved OS with CRT in all subsets (all P<.05). CRT with therapeutic doses (≥45 Gy) was associated with longer median survival than palliative doses (<45 Gy) and chemotherapy alone (24.9 vs 10.9 vs 15.6 months, respectively; P<.001). The benefit of CRT was present among not only those with distant lymph node metastasis (HR, 0.63; P=.04) but also those with distant organ disease (HR, 0.74; P<.001). Conclusions: In this large hypothesis-generating analysis, patients with newly diagnosed MAC who received definitive pelvic RT with chemotherapy lived significantly longer than those who received chemotherapy alone. Prospective trials evaluating definitive local RT for MAC are warranted.