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Effective anti-tumor immunity is driven by cytotoxic CD8 T cells with specificity for tumor antigens. However, the factors that control successful tumor rejection are not well understood. Here we identify a subpopulation of CD8 T cells that are tumor-antigen-specific and can be identified by KIR expression but paradoxically impair anti-tumor immunity in patients with melanoma. These tumor-antigen-specific KIRCD8 regulatory T cells target other tumor-antigen-specific CD8 T cells, can be detected in both the tumor and the blood, have a conserved transcriptional program and are associated with a poor overall survival. These findings broaden our understanding of the transcriptional and functional heterogeneity of human CD8 T cells and implicate KIRCD8 regulatory T cells as a cellular mediator of immune evasion in human cancer.