Although recombinant human bone morphogenetic protein-2 (rhBMP-2) is widely used to enhance fusion in posterior lumbar interbody fusion (PLIF), direct comparative studies at different rhBMP-2 application strategies remain limited. Therefore, this study aimed to compare the clinical efficacy of rhBMP-2 applied outside the cage and inside the cage in patients undergoing PLIF. A retrospective comparative study was conducted on patients who underwent one- or two-level PLIF. Patients were allocated into three groups according to graft materials: a control group without application of rhBMP-2, an extra-cage group receiving rhBMP-2 soaked in allogeneic bone grafts anteriorly located outside the cages, and an intra- cage group receiving rhBMP-2 soaked in collagen sponges located inside the cages. Radiographic fusion was evaluated at 1 year postoperatively using plain radiographs and computed tomography (CT). Clinical outcomes and postoperative complications were compared. CT-based fusion rates differed significantly among the three groups, with the highest fusion rate observed in the extra-cage group (80%), followed by the intra-cage group (60%) and the control group (52.2%) (p=0.024). Multivariate regression analysis revealed that the extra-cage application of rhBMP-2 was independently associated with higher odds of fusion compared with both no application (odds ratio [OR], 4.16; p=0.008) and the intra-cage application of rhBMP-2 (OR, 3.29; p=0.024). All groups exhibited significant improvement in clinical outcomes after surgery, with continued improvement over time; however, no significant differences were observed among the three groups at any time point after surgery. There were no significant differences among groups with regard to the postoperative complications, including heterotopic ossification, cage subsidence, osteolysis, and reoperation. In conclusion, extra-cage application of rhBMP-2 is safe and more efficient for fusion than intra-cage application of rhBMP-2 in PLIF.