Staphylococcus phage 812, strain K1/420, is a broad-acting bacteriophage infecting S. aureus. Phage 812 belongs to the family Herelleviridae and is closely related to kayviruses. We conducted a structural study of the viral particle of phage 812 before and after genome ejection. We characterized the protein components forming the particle and described the changes in their structural arrangement that govern genome anchoring, gating, and release.