Postoperative delirium (POD) is a frequent complication observed in patients following spinal surgeries. The incidence of POD is particularly concerning for major procedures like lumbar spinal fusion, leading to a range of detrimental outcomes. However, existing research on the prevalence and risk factors associated with POD after lumbar spinal fusion is limited, especially when relying on data from large-scale national databases. This study employed a comprehensive analysis of the National Inpatient Sample (NIS) database, encompassing data from 2010 to 2019. The study population included patients who underwent primary lumbar spinal fusion surgery. We compared patients who did not experience POD with those who developed POD postoperatively, analyzing demographic characteristics, pre-existing comorbidities, and perioperative complications. Analysis of the NIS database identified a total of 493,481 patients who underwent lumbar spinal fusion. The overall incidence of POD following the procedure was 1.07%. Patients who experienced delirium postoperatively exhibited significantly longer hospital stays, incurred higher healthcare costs, and faced a heightened risk of in-hospital mortality (p < 0.001). Additionally, POD following lumbar spinal fusion was associated with various medical complications, including acute renal failure, myocardial infarction, pneumonia, pulmonary embolism, stroke, and urinary tract infections. Several independent predictors were identified as being significantly associated with postoperative delirium, including advanced age (≥65 years old), pre-existing neurological disorders, a history of alcohol or drug abuse, depression, psychotic disorders, fluid and electrolyte imbalances, diabetes, weight loss, deficiency anemia, coagulopathy, congestive heart failure, pulmonary circulation disorders, peripheral vascular disease, chronic renal insufficiency, and receiving treatment at a teaching hospital. Notably, neurological disorders demonstrated the strongest correlation with the development of postoperative delirium. Overall, our analysis revealed a relatively low prevalence of POD following lumbar spinal fusion surgery. Nevertheless, it is critical to investigate and understand the independent predictors of POD to effectively prevent and mitigate its negative impact on patient outcomes.