Notch signalling is critical to help direct T‐cell lineage commitment in early T‐cell progenitors and in the development of αβ T‐cells. Epithelial and stromal cell populations in the thymus express the Notch DSL (Delta, Serrate and Lag2)ligands Delta‐like 1 (Dll1), Delta‐like 4 (Dll4), Jagged 1 and Jagged 2, and induce Notch signalling in thymocytes that express the Notch receptor. At present there is nothing known about the role of the Delta‐like 3 (Dll3) ligand in the immune system. Here we describe a novel cell autonomous role for Dll3 in αβ T‐cell development. We show that Dll3 cannot activate Notch when expressed in trans but like other Notch ligands it can inhibit Notch signalling when expressed in cis with the receptor. The loss of Dll3 leads to an increase in Hes5 expression in double positive thymocytes and their increased production of mature CD4+ and CD8+ T cells. Studies using competitive irradiation chimeras proved that Dll3 acts in a cell autonomous manner to regulate positive selection but not negative selection of autoreactive T cells. Our results indicate that Dll3 has a unique function during T‐cell development that is distinct from the role played by the other DSL ligands of Notch and is in keeping with other recent studies indicating that Dll1 and Dll3 ligands have non‐overlapping roles during embryonic development.