Thymocyte maturation is controlled by successive developmental checkpoints connected to the acquisition of a functional T cell receptor (TCR). During thymocyte selection, engagement of the TCRregulates the fine balance between death and survival signals. At the final stages of single‐positive (SP) thymocyte maturation, the coupling of the TCR changes from death‐ to proliferation‐inducing signals, a competence required for optimal effector functions in the periphery. We show here that in RelB mutant thymuses, thymocyte differentiation of CD24^– SP cells is partially impaired. Competitive bone marrow reconstitution experiments show that this defect is constitutive to the lymphoid compartment. This is accompanied by an increased proportion of apoptotic thymocytes and a drastically reduced proliferation upon activation with anti‐CD3 antibody/PMA stimulation. Thus, the RelB protein contributes to the quality of cell signaling in thymocytes by providing anti‐apoptotic signals. These results suggest that in addition to its major role on the activation of antigen‐presenting cell function, the RelB protein is intrinsically required for terminal thymocyte differentiation and activation.