Adoptive cell therapy (ACT) with ex-vivo expanded tumor-infiltrating lymphocytes (TILs, TIL-ACT) has shown clinical efficacy in a significant proportion of patients with metastatic melanoma. To further target TIL-ACT towards responsive patients, identifying predictive biomarkers and understanding broader immune dynamics remain critical. This study investigated the peripheral blood immune landscape in 47 patients with metastatic melanoma undergoing TIL-ACT, assessing antitumor reactivity and peripheral immune cell profiles before and after treatment. Responders displayed increased frequency of circulating tumor-reactive cells post-treatment, and higher baseline levels of activated CD57-expressing T cells, serving as potential biomarkers of response. In contrast, persistent high serum levels of interleukin (IL)-6 and IL-8, higher frequencies of CD38-expressing T cells and regulatory T cells (Tregs) post-treatment, correlated with unfavorable outcomes. These findings contribute to understanding of the peripheral immune landscape associated with response to TIL-ACT, offering valuable insights into predictive biomarkers and mechanisms to improve patient selection.