Several approaches for horizontal ridge augmentation have been proposed, including guided bone regeneration (GBR), ridge split, and block grafts. Minimally invasive techniques for horizontal GBR have been introduced to reestablish an adequate bone volume, minimizing tissue trauma and patient morbidity. The present article describes a tunnel technique with a subperiosteal bag for horizontal GBR. A collagen membrane is partially perforated, folded, and sutured to form a bag that is filled with xenogeneic bone graft. The filled bag is inserted into a subperiosteal tunnel such that the perforated side faces the alveolar ridge and the nonperforated side faces the tunnel flap. The main advantage of this approach is the preservation of the periosteum and the enhanced blood supply to the flap, which may contribute to increased favorable wound healing and a reduced risk of flap dehiscence and membrane exposure. This novel tunnel approach for horizontal GBR using a customized bag made from a collagen membrane, specifically adapted and filled with deproteinized bovine bone, resulted in a significant ridge volume gain that allowed implant placement.