Background: Chemotherapy has the potential to induce CD8+ T-cell infiltration in the tumor microenvironment (TME) and activate the anti-tumor immune response in several cancers including esophageal squamous cell carcinoma (ESCC). The tumor cell-intrinsic cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathway has been known as a critical component for regulating immune cell activation in the TME. However, its effect on the infiltration of immune cells induced by chemotherapy in the ESCC TME has not been investigated.Methods: We examined the effect of the tumor-cell intrinsic cGAS–STING pathway on the infiltration of CD8+ T cells induced by chemotherapy in ESCC using ESCC cell lines and surgically resected ESCC specimens from patients who received neoadjuvant chemotherapy (NAC).Results: We found that chemotherapeutic agents, including 5-fluorouracil (5-FU) and cisplatin (CDDP), activated the cGAS–STING pathway, consequently inducing the expression of type I interferon and T-cell-attracting chemokines in ESCC cells. Moreover, the tumor cell-intrinsic expression of cGAS–STING was significantly and positively associated with the density of CD8+ T cells in ESCC after NAC. However, the tumor cell-intrinsic expression of cGAS–STING did not significantly impact clinical outcomes in patients with ESCC after NAC.Conclusion: Our findings suggest that the tumor cell-intrinsic cGAS–STING pathway might contribute to chemotherapy-induced immune cell activation in the ESCC TME.