Fertilization is an essential biological process in sexual reproduction and comprises a series of molecular interactions between the sperm and egg1,2. The fusion of the haploid spermatozoon and oocyte is the culminating event in mammalian fertilization, enabling the creation of a new, genetically distinct diploid organism3,4. The merger of two gametes is achieved through a two-step mechanism in which the sperm protein IZUMO1 on the equatorial segment of the acrosome-reacted sperm recognizes its receptor, JUNO, on the egg surface4,5,6. This recognition is followed by the fusion of the two plasma membranes. IZUMO1 and JUNO proteins are indispensable for fertilization, as constitutive knockdown of either protein results in mice that are healthy but infertile5,6. Despite their central importance in reproductive medicine, the molecular architectures of these proteins and the details of their functional roles in fertilization are not known. Here we present the crystal structures of human IZUMO1 and JUNO in unbound and bound conformations. The human IZUMO1 structure exhibits a distinct boomerang shape and provides structural insights into the IZUMO family of proteins7. Human IZUMO1 forms a high-affinity complex with JUNO and undergoes a major conformational change within its N-terminal domain upon binding to the egg-surface receptor. Our results provide insights into the molecular basis of sperm–egg recognition, cross-species fertilization, and the barrier to polyspermy, thereby promising benefits for the rational development of non-hormonal contraceptives and fertility treatments for humans and other mammals.