Abstract Although several cancer immunotherapy strategies are based on the use of analog peptides and on the modulation of the TCR affinity of adoptively transferred T cells, it remains unclear whether tumor-specific T cell activation by strong and weak TCR stimuli evoke different Ca2+ signatures from the Ca2+ intracellular stores and whether the amplitude of Ca2+ release from the endoplasmic reticulum (ER) can be further modulated by coreceptor binding to peptide/MHC. In this study, we combined functional, structural, and kinetic measurements to correlate the intensity of Ca2+ signals triggered by the stimulation of the 1G4 T cell clone specific to the tumor epitope NY-ESO-1157–165. Two analogs of the NY-ESO-1157–165 peptide, having similar affinity to HLA-A2 molecules, but a 6-fold difference in binding affinity for the 1G4 TCR, resulted in different Ca2+ signals and T cell activation. 1G4 stimulation by the stronger stimulus emptied the ER of stored Ca2+, even in the absence of CD8 binding, resulting in sustained Ca2+ influx. In contrast, the weaker stimulus induced only partial emptying of stored Ca2+, resulting in significantly diminished and oscillatory Ca2+ signals, which were enhanced by CD8 binding. Our data define the range of TCR/peptide MHC affinities required to induce depletion of Ca2+ from intracellular stores and provide insights into the ability of T cells to tailor the use of the CD8 coreceptor to enhance Ca2+ release from the ER. This, in turn, modulates Ca2+ influx from the extracellular environment, ultimately controlling T cell activation.