The present prospective case series evaluated the efficacy, the intra- and postsurgical complications, and the patient-related outcomes of a tunnel technique with a subperiosteal bag (TTSB) for horizontal ridge augmentation prior to implant placement. Twenty-five systemically healthy patients searching for implant rehabilitation and presenting with a horizontal ridge deficiency requiring bone augmentation before implant placement were included. Guided bone regeneration (GBR) was performed using the TTSB technique, involving the preparation of a collagen membrane that is perforated, folded, and sutured to form a bag that is then filled with a xenogeneic bone graft material. The bag is then inserted into a subperiosteal tunnel. Horizontal bone gain (HBG) was calculated 6 months after the surgery using CBCT. Healing was uneventful and with minimal morbidity. Average HBG was 3.7 ± 1.8 mm (P < .05), with a vertical component gain of 1.3 ± 1.7 mm (P > .05). In 12 patients, additional bone augmentation was required at the time of implant placement. A higher HBG was achieved in the maxilla than in the mandible (4.3 ± 1.3 mm vs 3.2 ± 1.8 mm, respectively; P < .05), and sites with an initial horizontal ridge width ≤ 3.5 mm were associated with a higher chance of requiring additional bone grafting at the time of implant placement compared to sites with an initial horizontal ridge width > 3.5 mm. The present case series demonstrated that TTSB is an effective approach for ridge augmentation prior to implant placement with minimal intra- and postoperative complications or patient morbidity.