Selection of T-cell repertoire is determined by the binding propensity of the T-cell receptor (TCR) to self-ligands in the thymus. Only thymocytes whose TCRs bind self peptide-major histocompatibility complex (pMHC) with appropriate avidities are thought to be selected for survival (positive selection) while those that bind self-pMHC too strongly will be purged to limit autoimmunity (negative selection). However, binding quantified by three-dimensional (3D) measures such as tetramer staining, photoaffinity labeling or surface plasmon resonance have yielded limited success in predicting selection outcomes. Here we show that force-regulated dissociation kinetics of TCR and CD8 from pMHC determines thymocyte selection. Without force, our two-dimensional assays provided sufficient sensitivity to identify positive selecting ligands unmeasurable by 3D assays, but neither the TCR–pMHC bimolecular affinity nor the TCR–pMHC–CD8 trimolecular avidity could completely discern selection outcomes. Instead, force distinguished all outcomes through induction of cooperative binding to convert the two TCR–pMHC and MHC–CD8 slip bonds into a catch-slip bond of TCR–pMHC–CD8 interaction for negative but not positive selecting ligands. Because force prolongs lifetimes of catch bonds but shortens lifetimes of slip bonds, the differential numbers of TCRs and CD8s engaged by negative and positive selecting ligands surviving a given time increase with force and reach a maximum at 10-15pN. Our results reveal the mechanism driving thymocyte selection highlighting the roles of force and coreceptor.