Blocking the immunosuppressive function of T-cell immunoglobulin mucin-3 (TIM-3) is an established therapeutic strategy to maximize the efficacy of immune checkpoint inhibitors for cancer immunotherapy. Currently, effective inhibition of TIM-3 interactions relies on monoclonal antibodies (mAbs), which come with drawbacks such as immunogenicity risk, limited tumor penetration, and high manufacturing costs. Guided by the X-ray cocrystal structures of TIM-3 with mAbs, we report an structure-based rational design of constrained peptides as potent TIM-3 inhibitors. The top cyclic peptide from our study () binds TIM-3 with a value of 166.3 ± 12.1 nM as determined by surface plasmon resonance (SPR) screening. Remarkably, efficiently inhibits key TIM-3 interactions with natural TIM-3 ligands at submicromolar concentrations in a panel of cell-free and cell-based assays. The capacity of to reverse immunosuppression in T-cell/cancer cell cocultures, coupled with favorable pharmacokinetic properties, highlights the potential of for further evaluation in preclinical models of immuno-oncology.