Abstract Therapeutic strategies using anti–PD-1–blocking antibodies reported unparalleled effectiveness for melanoma immunotherapy, but deciphering immune responses modulated by anti–PD-1 treatment remains a crucial issue. Here, we analyzed the composition and functions of the large Melan-A–specific T-cell repertoire in the peripheral blood of 9 melanoma patients before and after 2 months of treatment with anti–PD-1. We observed amplification of Melan-A–specific Vß subfamilies undetectable before therapy (thereafter called emerging Vß subfamilies) in responding patients, with a predominant expansion in patients with a complete response. These emerging Vß subfamilies displayed a higher functional avidity for their cognate antigen than Vß subfamilies not amplified upon anti–PD-1 therapy and could be identified by a sustained coexpression of PD-1 and TIGIT receptors. Thus, in addition to the emergence of neoantigen-specific T cells previously documented upon anti–PD-1 therapy, our work describes the emergence of high-avidity Melan-A–specific clonotypes as a surrogate marker of treatment efficacy. Cancer Res; 77(24); 7083–93. ©2017 AACR.