OBJECTIVE Frailty—the state defined by decreased physiological reserve and increased vulnerability to physiological stress—is exceedingly common in oncology patients. Given the palliative nature of spine metastasis surgery, it is imperative that patients be healthy enough to tolerate the physical insult of surgery. In the present study, the authors compared the association of two frailty metrics and the widely used Charlson Comorbidity Index (CCI) with postoperative morbidity in spine metastasis patients. METHODS A retrospective cohort of patients who underwent operations for spinal metastases at a comprehensive cancer center were identified. Data on patient demographic characteristics, disease state, medical comorbidities, operative details, and postoperative outcomes were collected. Frailty was measured with the modified 5-item frailty index (mFI-5) and metastatic spinal tumor frailty index (MSTFI). Outcomes of interest were length of stay (LOS) greater than the 75th percentile of the cohort, nonroutine discharge, and the occurrence of ≥ 1 postoperative complication. RESULTS In total, 322 patients were included (mean age 59.5 ± 12 years; 56.9% of patients were male). The mean ± SD LOS was 11.2 ± 9.9 days, 44.5% of patients had nonroutine discharge, and 24.0% experienced ≥ 1 postoperative complication. On multivariable analysis, increased frailty on mFI-5 and MSTFI was independently predictive of all three outcomes: prolonged LOS (OR 1.67 per point, 95% CI 1.06–2.63, p = 0.03; and OR 1.63 per point, 95% CI 1.29–2.05, p < 0.01, respectively), nonroutine discharge (OR 2.65 per point, 95% CI 1.74–4.04, p < 0.01; and OR 1.69 per point, 95% CI 1.36–2.11, p < 0.01), and ≥ 1 complication (OR 1.95 per point, 95% CI 1.23–3.09, p = 0.01; and OR 1.41 per point, 95% CI 1.12–1.77, p < 0.01). CCI was found to be independently predictive of only the occurrence of ≥ 1 postoperative complication (OR 1.45 per point, 95% CI 1.22–1.72, p < 0.01). CONCLUSIONS Frailty measured with either mFI-5 or MSTFI scores was a more robust independent predictor of adverse postoperative outcomes than the more widely used CCI. Both mFI-5 and MSTFI were significantly associated with prolonged LOS, higher complication rates, and nonroutine discharge. Further investigation in a prospective multicenter cohort is merited.