T cell factor‐1 (TCF‐1) (encoded by the TCF7 gene) is a transcription factor that plays important role during the T cell development and differentiation for T cell to exercise its functions including producing memory T cells. Not only TCF‐1 can modulate the T cell development but also exerts various effects on the differentiation and function of mature CD8⁺ T cells. In addition, it drives the production and maintenance of the immune response of CD8⁺ T cells after PD‐1 checkpoint blockade therapy. TCF‐1 can serve as a potential target of immunotherapy and may provide promising novel treatment strategies for patients with cancer and infections. Moreover, TCF‐1 is a potential biomarker of CD8⁺ T cell functionality to predict the efficacy of immunotherapy in fighting against cancer and infections. Herein, we summarize the role of TCF‐1 in T cell development and its applications in the treatment of cancer and infectious diseases.