Abstract Noncovalent bonds typically mediate interactions between a T cell receptor (TCR) and a peptide-major histocompatibility complex (pMHC) ligand. In this study, we describe a novel mechanism involving a covalent TCR-pMHC interaction through a cysteine-cysteine disulfide bond. By introducing cysteine resodues into the TCR CDR3 and into the peptide of a known TCR-pMHC combination, we show that this disulfide bond formation does not require structural rearrangement and can still occur even when the initial affinity of the TCR-pMHC interaction is low. This mechanism strongly activates TCR signaling and has potential applications in basic immunology to further our fundamental understanding of T cell activation/differentiation as well as applied clinically for improving the efficacy and sensitivity of adoptive T cell therapies. Our findings shed new light on T cell activation and provide additional avenues for the development of modern immunotherapy.