The present study investigated the treatment and survival outcomes of patients with synchronous primary esophageal squamous cell carcinoma and gastric adenocarcinoma. The medical records of 10,783 patients with primary esophageal squamous cancer treated at our institution between 1995 and 2012 were retrospectively reviewed. Overall survival (OS) rates were calculated using the Kaplan-Meier method. The incidence was 0.38% (41/10,783). Of these 41 patients, 26 underwent curative surgery, ten received palliative chemotherapy or radiotherapy, and five received no treatment. The median OS of the surgery, palliative-therapy, and treatment-free groups was 17.1, 9.0, and 3.8 months, respectively. The 1-, 3-, 5-, and 10-year OS rates for the surgery group were 77%, 45%, 33%, and 19%, respectively. No significant differences in median OS were observed between the surgery group and the historical cohort of isolated esophageal cancer (n = 186) (17.1 vs. 21.0 months, P = 0.061) or isolated gastric cancer (n = 51) (17.1 vs. 28.9 months, P = 0.875), or between the palliative-therapy group and its corresponding historical cohort (n = 30) (9.0 vs. 8.3 months, P = 0.862). The survival outcomes of patients with synchronous primary esophageal squamous and gastric cancers were not worse than those of patients with isolated esophageal cancer or isolated gastric cancer.