Abstract: Endoplasmic reticulum (ER) stress is involved in the development and progression of tumors.In recent years, great attention has been paid to the study of the interplay of ER stress and T cell differentiation and functionality.Intense ER stress in the tumor-infiltrating T cells exacerbates T cell exhaustion and impairs T cell anti-tumor immunity.Therefore, a variety of ER stress inhibitors have been developed and utilized to alleviate T cell exhaustion, which improves T cell function in tumor microenvironment.Furthermore, the downregulation of several circadian clock genes like Per1 and Per2 also aggravates T cell exhaustion, and the key downstream effector molecules in ER stress regulate the transcription of Per family, thus enhancing the T cell function.In the present manuscript, we particularly summarize how ER stress impacts the anti-tumor immunity of T cells, and further discuss potential strategies for improving tumor immunotherapy via targeting ER stress.