Purpose <p>The aim of this study was to investigate the prognostic impact of thoracic extra-regional lymph node metastasis (M1b-LYM, determined by the 12th edition of the Japanese Classification of Esophageal Cancer) in definitive radiotherapy for esophageal squamous cell carcinoma.</p> Methods <p>Eighty-six consecutive patients who underwent definitive radiotherapy for esophageal squamous cell carcinoma between 2017 and 2022 at our institute were included in this retrospective study. Progression-free survival (PFS), overall survival (OS), distant metastasis (DM), and loco-regional recurrence (LR) were compared between patients with and without M1b-LYM using Kaplan–Meier or cumulative incidence function analysis.</p> Results <p>Among the 86 patients, 15 had M1b-LYM metastasis (the M1b( +) group) and 71 had no M1b-LYM metastasis (the M1b(-) group). The median follow-up period was 38&#xa0;months. The 2-year PFS, OS, cumulative incidence of DM, and cumulative incidence of LR for the M1b(-) group versus M1b( +) group were 41% vs 20% (p = 0.129), 58% vs 47% (p = 0.172), 31% vs 33% (p = 0.906), and 31% vs 60% (p = 0.0369), respectively. Multivariate analysis showed that M1b( +) was associated with higher LR (p = 0.0350), T stage was associated with poorer PFS (p = 0.0138), and omitting chemotherapy was associated with poorer PFS (p = 0.0160) and OS (p &lt; 0.01).</p> Conclusion <p>The presence of thoracic extra-regional lymph node metastasis was associated with poor loco-regional control but not distant metastasis or survival in esophageal squamous cell carcinoma patients after definitive radiotherapy.</p> Trial registration number <p>This study was retrospectively registered on 21 June 2024 (R06-053).</p>