Nectin and nectin-like (NECL) adhesion molecules are broadly overexpressed in a wide range of cancers. By binding to these adhesion molecules, the immunoreceptors DNAX accessory molecule-1 (DNAM-1), CD96 molecule (CD96), and T cell immunoreceptor with Ig and ITIM domains (TIGIT) play a crucial role in regulating the anticancer activities of immune effector cells. However, within this axis, it remains unclear how DNAM-1 recognizes its cognate ligands. Here, we determined the structure of human DNAM-1 in complex with nectin-like protein-5 (NECL-5) at 2.8 Å resolution. Unexpectedly, we found that the two extracellular domains (D1–D2) of DNAM-1 adopt an unconventional "collapsed" arrangement that is markedly distinct from those in other immunoglobulin-based immunoreceptors. The DNAM-1:NECL-5 interaction was underpinned by conserved lock-and-key motifs located within their respective D1 domains, but also included a distinct interface derived from DNAM-1 D2. Mutation of the signature DNAM-1 "key" motif within the D1 domain attenuated NECL-5 binding and natural killer cell–mediated cytotoxicity. Altogether, our results have implications for understanding the binding mode of an immune receptor family that is emerging as a viable candidate for cancer immunotherapy.