Signaling via the T cell antigen receptor (TCR) during the CD4⁺CD8⁺ double-positive developmental stage determines thymocyte selection and lineage commitment. Here we describe a previously uncharacterized T cell–expressed protein, Tespa1, with critical functions during the positive selection of thymocytes. Tespa1^−/− mice had fewer mature thymic CD4⁺ and CD8⁺ T cells, which reflected impaired thymocyte development. Tespa1 associated with the TCR signaling components PLC-γ1 and Grb2, and Tespa1 deficiency resulted in attenuated TCR signaling, as reflected by defective activation of the Erk–AP-1 and Ca²⁺-NFAT pathways. Our findings demonstrate that Tespa1 is a component of the TCR signalosome and is essential for T cell selection and maturation through the regulation of TCR signaling during T cell development.