microRNAs are short noncoding RNAs that regulate protein expression posttranscriptionally. We previously showed that miR-155 promotes effector CD8 ⁺ T-cell responses. However, little is known about the regulation of miR-155 expression. Here, we report that antigen affinity and dose determine miR-155 expression in CD8 ⁺ T cells. In B16 tumors expressing a low-affinity antigen ligand, tumor-specific infiltrating CD8 ⁺ T cells showed variable miR-155 expression, whereby high miR-155 expression was associated with more cytokine-producing cells and tumor control. Moreover, anti-PD-1 treatment led to both increased miR-155 expression and tumor control by specific CD8 ⁺ T cells. In addition, miR-155 overexpression enhanced exhausted CD8 ⁺ T-cell persistence in the LCMV cl13 chronic viral infection model. In agreement with these observations in mouse models, miR-155 expression in human effector memory CD8 ⁺ T cells positively correlated with their frequencies in tumor-infiltrated lymph nodes of melanoma patients. Low miR-155 target gene signature in tumors was associated with prolonged overall survival in melanoma patients. Altogether, these results raise the possibility that high miR-155 expression in CD8 ⁺ tumor-infiltrating T cells may be a surrogate marker of the relative potency of in situ antigen-specific CD8 ⁺ T-cell responses.