Background: Adjacent segment disease (ASD) is a well‑recognised long‑term complication of lumbar spinal fusion, with the L5/S1 segment being particularly vulnerable when fusion terminates at L5. Data from resource‑constrained Nigerian centres on the incidence, features, and outcomes of L5/S1 ASD are scarce. This study evaluated the specific features, outcomes, and predictors of L5/S1 ASD following lumbar spine surgery at a Nigerian tertiary centre. Methods: A retrospective cohort study of 311 patients who underwent lumbar spine surgery with fixation terminating at L5 at the National Orthopaedic Hospital Dala, Kano, from January 2019 to June 2025 was conducted. Sixty‑three patients (20.3%) developed ASD over a minimum 1‑year follow‑up. Outcomes assessed included specific ASD features, management strategy (surgical vs conservative), patient satisfaction, and predictors of ASD and satisfaction. Results: The most common ASD features were disc degeneration (82.5%), facet joint arthropathy (68.3%), spinal stenosis (55.6%), and spondylolisthesis (36.5%). Twenty‑eight patients (44.4%) underwent surgical extension to S1, with 3 (4.8%) requiring additional S2AI screw fixation. The remaining 35 patients (55.6%) were managed conservatively. Surgical patients had significantly better pain relief (VAS 2.8±1.4 vs 4.6±1.8, p<0.001) and functional outcomes (ODI 28.4±12.6 vs 42.6±15.2, p<0.001). Patient satisfaction was reported by 48 patients (76.2%), with surgical management (OR 3.2, 95% CI: 1.4–7.3) and shorter symptom duration (OR 2.6, 95% CI: 1.1–6.1) being independent predictors of satisfaction. Predictors of ASD included: fusion length ≥3 levels (OR 3.4, 95% CI: 1.6–7.2), preoperative disc degeneration at L5/S1 (OR 2.8, 95% CI: 1.3–6.1), and age >55 years (OR 2.4, 95% CI: 1.1–5.2). Conclusions: L5/S1 ASD occurs in 20.3% of patients following lumbar fusion terminating at L5. Surgical extension to S1 with or without S2AI screws provides superior outcomes compared with conservative management. Fusion length, preoperative L5/S1 degeneration, and older age are significant predictors of ASD.