Many promising targets for T-cell-based cancer immunotherapies are self-antigens. During thymic selection, T cells bearing T cell receptors (TCRs) with high affinity for self-antigen are eliminated. The affinity of the remaining low-avidity TCRs can be improved to increase their antitumor efficacy, but conventional saturation mutagenesis approaches are labor intensive, and the resulting TCRs may be cross-reactive. Here we describe the in vitro maturation and selection of mouse and human T cells on antigen-expressing feeder cells to develop higher-affinity TCRs. The approach takes advantage of natural Tcrb gene rearrangement to generate diversity in the length and composition of CDR3beta. In vitro differentiation of progenitors transduced with a known Tcra gene in the presence of antigen drives differentiation of cells with a distinct agonist-selected phenotype. We purified these cells to generate TCRbeta chain libraries pre-enriched for target antigen specificity. Several TCRbeta chains paired with a transgenic TCRalpha chain to produce a TCR with higher affinity than the parental TCR for target antigen, without evidence of cross-reactivity.