The transcription factor TCF-1 is essential for the development and function of regulatory T (T) cells; however, its function is poorly understood. Here, we show that TCF-1 primarily suppresses transcription of genes that are co-bound by Foxp3. Single-cell RNA-sequencing analysis identified effector memory T cells and central memory T cells with differential expression of Klf2 and memory and activation markers. TCF-1 deficiency did not change the core T cell transcriptional signature, but promoted alternative signaling pathways whereby T cells became activated and gained gut-homing properties and characteristics of the T17 subset of helper T cells. TCF-1-deficient T cells strongly suppressed T cell proliferation and cytotoxicity, but were compromised in controlling CD4 T cell polarization and inflammation. In mice with polyposis, T cell-specific TCF-1 deficiency promoted tumor growth. Consistently, tumor-infiltrating T cells of patients with colorectal cancer showed lower TCF-1 expression and increased T17 expression signatures compared to adjacent normal tissue and circulating T cells. Thus, T cell-specific TCF-1 expression differentially regulates T17-mediated inflammation and T cell cytotoxicity, and can determine colorectal cancer outcome.