Bone graft extenders are widely used to supplement local autograft in posterior spinal fusion for adolescent idiopathic scoliosis (AIS). P-15, a novel synthetic 15-amino-acid polypeptide, has shown promise in promoting bone healing. This study compares P-15 (i-Factor) as a bone graft extender to allograft in adolescents undergoing segmental pedicle screw instrumentation for AIS. We conducted a retrospective analysis of prospectively collected data on 60 adolescents with AIS who underwent segmental pedicle screw instrumentation with a minimum 2-year follow-up. In this preliminary study, 30 patients received P-15, and 30 received an allograft in addition to a local autograft. Operative time, radiographic outcomes, and health-related quality of life were assessed. Plain radiographs were used to evaluate nonunion. Operative time was significantly shorter in the P-15 group (2.7 vs. 3.4 h; P = 0.01). At 2-year follow-up, the mean major curve was 17° (SD: 6.2) in the P-15 group and 20° (SD: 5.2) in the allograft group (P = 0.057). The mean loss of correction was similar between the groups (3.3 vs. 1.2°; P = 0.092). No cases of nonunion or deep surgical site infections were observed. Health-related quality of life scores were comparable between the groups. P-15 is, according to the current study, a safe and effective bone graft extender for posterior spinal fusion in adolescents with AIS, demonstrating similar radiographic and clinical outcomes compared to allograft while reducing operative time. Further research is warranted to assess the long-term results with P15 in adolescents undergoing pedicle screw instrumentation for idiopathic scoliosis.