Background Data: Posterior lumbar decompression combined with instrumented posterior lumbar interbody fusion (PLIF) is the gold standard management of adult spondylolisthesis. Complete anatomic surgical reduction of slippage, although controversial, is associated with encouraging results. Purpose: This study aims to assess the clinical outcome of complete open surgical reduction and instrumented PLIF in adult single-level lumbar spondylolisthesis. Study design: Retrospective clinical case study. Patients and Methods: Retrospective review of standing data and radiographs of adult patients with single-level lumbar spondylolisthesis (regardless of the aetiology) from January 2018 to January 2020. All patients have undergone posterior lumbar decompression, pedicle screw instrumentation, complete reduction of the slippage, and PLIF. All patients were assessed at six and 12 months postoperatively by Visual Analogue Scale (VAS), Oswestry Disability Index (ODI), and the Odom criteria. Results: A total of 118 patients met the inclusion criteria. There was no neurologic deterioration in any of the patients postoperatively. There was a significant change in VAS, ODI, and Odom’s criteria at six and 12 months postoperatively (p < 0.001). The mean value for VAS and ODI were 0.61 ± 1.03 and 4.52 ± 7.02, respectively, at the last follow-up. There was a significant change in Odom’s criteria as 61.9% of patients had excellent outcomes at the last follow-up. Conclusion: Complete surgical reduction of single-level lumbar spondylolisthesis combined with instrumented PLIF offers adequate neural element decompression and satisfactory clinical outcome. (2020ESJ227)