Purpose This study evaluates the influence of patient frailty status on postoperative complications in those receiving single-level lumbar fusion surgery. Methods The nationwide readmission database was retrospectively queried between 2016 and 2017 for all patients receiving single-level lumbar fusion surgery. Readmissions were analyzed at 30, 90, and 180 days from primary discharge. Demographics, frailty status, and relevant complications were queried at index admission and all readmission intervals. Complications of interest included infection, urinary tract infection (UTI), posthemorrhagic anemia, inpatient length of stay (LOS), and adjusted all-payer costs. Nearest-neighbor propensity score matching for demographics was implemented to identify non-frail control patients with similar diagnoses and procedures. The analysis used nonparametric Mann–Whitney U testing and odds ratios. Results Comparing propensity-matched cohorts revealed significantly greater LOS and total all-payer inpatient costs in frail patients than non-frail patients with comparable demographics and comorbidities ( p  < 0.0001 for both). Furthermore, frail patients encountered higher rates of UTI (OR: 3.97, 95%CI: 3.21–4.95, p  < 0.0001), infection (OR: 6.87, 95%CI: 4.55–10.86, p  < 0.0001), and posthemorrhagic anemia (OR: 1.94, 95%CI: 1.71–2.19, p  < 0.0001) immediately following surgery. Frail patients had significantly higher rates of 30-day (OR: 1.24, 95%CI: 1.02–1.51, p  = 0.035), 90-day (OR: 1.38, 95%CI: 1.17–1.63, p  < 0.001), and 180-day (OR: 1.55, 95%CI: 1.30–1.85, p  < 0.0001) readmissions. Lastly, frail patients had higher rates of infection at 30-day (OR: 1.61, 95%CI: 1.05–2.46, p  = 0.027) and 90-day (OR: 1.51, 95%CI: 1.07–2.16, p  = 0.020) readmission intervals. Conclusions Patient frailty status may serve as an important predictor of postoperative outcomes in patients receiving single-level lumbar fusion surgery.