Double fertilization in angiosperms, a simultaneous fusion of sperm cells with the egg and central cells, requires precise gamete recognition and membrane fusion mediated by sperm cell surface proteins. However, the molecular regulation of these interactions remains incompletely understood. Here, we identify GEX3, a sperm-expressed transmembrane protein with an extracellular β-propeller domain, as a critical mediator of gamete adhesion and fusion. Loss of GEX3 disrupts fertilization by impairing sperm attachment to and fusion with female gametes. GEX3 physically and genetically interacts with GEX2 and DMP8/9 to promote the egg-cell-triggered trafficking of the fusogen HAP2/GCS1 to the sperm plasma membrane. Our findings establish GEX3 as an important component of the sperm cell surface machinery for coordinated membrane adhesion and fusion during double fertilization in flowering plants.