To aid clinicians strategizing treatment for upper esophageal squamous cell carcinoma (ESCC), this retrospective study investigated associations between primary gross tumor volume (GTVp) and prognosis in patients given surgical resection, radiotherapy, or both resection and radiotherapy.The population comprised 568 patients with upper ESCC given definitive treatment, including 238, 216, and 114 who underwent surgery, radiotherapy, or combined radiotherapy and surgery. GTVp as a continuous variable was entered into the multivariate Cox model using penalized splines (P-splines) to determine the optimal cutoff value. Propensity score matching (PSM) was used to adjust imbalanced characteristics among the treatment groups.P-spline regression revealed a dependence of patient outcomes on GTVp, with 30 cm3 being an optimal cut-off for differences in overall and progression-free survival (OS, PFS). GTVp ≥30 cm3 was a negative independent prognostic factor for OS and PFS. PSM analyses confirmed the prognostic value of GTVp. For GTVp < 30 cm3, no significant survival differences were observed among the 3 treatments. For GTVp ≥30 cm3, the worst 5-year OS rate was experienced by those given surgery. The 5-year PFS rate of patients given combined radiotherapy and surgery was significantly better than that of patients given radiotherapy. The surgical complications of patients given the combined treatment were comparable to those who received surgery, but radiation side effects were significantly lower.GTVp is prognostic for OS and PFS in upper ESCC. For patients with GTVp ≥30 cm3, radiotherapy plus surgery was more effective than either treatment alone.© 2021. The Author(s).