BACKGROUND: As global populations age, the number of nonagenarians admitted to intensive care units (ICUs) is rising. Frailty, a multidimensional syndrome marked by diminished physiological reserves, has been associated with adverse outcomes in older ICU patients. However, evidence remains limited regarding its prognostic significance in nonagenarians, who represent a unique and rapidly growing subset of critically ill patients. This study aimed to evaluate the impact of frailty on in-hospital mortality and length of stay among nonagenarian ICU patients in Australia and New Zealand. METHODS: We conducted a retrospective cohort study using data from the ANZICS Adult Patient Database, including nonagenarians admitted to 211 ICUs between 2017 and 2023 with documented Clinical Frailty Scale (CFS) scores. Patients were classified as frail (CFS ≥ 5) or non-frail (CFS < 5). Propensity score matching (1:1) was applied to adjust for confounders including age, sex, illness severity, admission type, and comorbidities. Outcomes included ICU and hospital mortality, and ICU and hospital lengths of stay (LOS). Statistical analyses included multivariable Cox regression, log-transformed logistic regression, and Fine Gray competing risks models. RESULTS: Among 16,439 nonagenarians, 8220 patients were propensity matched. In the matched cohort, frailty was independently associated with increased hospital mortality (adjusted HR 1.352, 95% CI 1.192-1.534, p < 0.001) and ICU mortality (adjusted HR 1.242, 95% CI 1.044-1.440, p = 0.017). Each one-point increase in CFS score was associated with a 9% increase in the odds ratio of ICU mortality (OR 1.09, 95% CI 1.01-1.18, p = 0.026) and a 19% increase in the odds ratio of hospital mortality (OR 1.19, 95% CI 1.10-1.28, p < 0.001). Frailty was not associated with ICU LOS after adjustment (p = 0.739) but predicted prolonged hospital LOS (adjusted β = 1.051, 95% CI 1.033-1.070, p < 0.001). CONCLUSIONS: Frailty is a strong, independent predictor of hospital mortality and prolonged hospitalization in critically ill nonagenarians, even after adjusting for illness severity and comorbidities. These findings support the incorporation of frailty assessment into early risk stratification and clinical decision-making in ICU settings, to facilitate goal-concordant care and optimize resource allocation for the very elderly.