This retrospective comparative study investigated whether adding a lateral interbody release technique to L5/S1 posterior lumbar interbody fusion (PLIF) enhances sagittal alignment, intervertebral restoration, and reliable interbody fusion in adult spinal deformity (ASD) surgery. A total of 110 consecutive patients who underwent long-segment fusion extending from the thoracic spine to the pelvis between 2013 and 2021 were included. Patients were allocated to a non-release group (n = 54) or a release group (n = 56) according to the surgical period and operative strategy. Standing radiographs were obtained preoperatively and at 2 years postoperatively to assess lumbar lordosis and sagittal vertical axis. Fusion status and lateral bridging callus (LBC) formation at the L5/S1 level were evaluated using computed tomography and classified according to the Berjano system, with complete or stable union defined as successful fusion. Postoperative lumbar lordosis was significantly greater and sagittal vertical axis was significantly smaller in the release group compared with the non-release group. Segmental lordosis at L5/S1 increased to a greater extent in the release group, accompanied by significantly larger postoperative anterior and posterior disc heights and a greater magnitude of disc height restoration. Endplate injury was observed less frequently in the release group. Furthermore, LBC formation occurred more often, and the overall fusion rate at the lumbosacral junction was significantly higher in patients who underwent the lateral interbody release technique. No neurological, vascular, or approach-related complications attributable to the release maneuver were identified during the follow-up period. These results indicate that lateral interbody release during L5/S1 PLIF reproduces key biomechanical advantages associated with lateral lumbar interbody fusion, including more complete annular release, improved segmental lordosis, and enhanced peripheral bone formation. This posteriorly applied technique may represent a safe, reproducible, and clinically meaningful strategy for optimizing sagittal alignment and promoting durable fusion at the lumbosacral junction in complex ASD surgery.