Objective: Cerebral edema significantly impacts the functional outcomes in patients with acute stroke treated with thrombectomy, especially those with an extended time window (6-24 hours). This study was to investigate whether pre-thrombectomy cerebral edema predicts functional prognosis in ischemic stroke patients within an extended onset time window. Methods: All patients from Xuanwu Hospital of Capital Medical University underwent computed tomography (CT) examination and endovascular treatment between December 2021 and December 2023. Quantitative Net Water Uptake (NWU) was assessed according to baseline CT. The ability to predict onset time and outcomes was assessed by univariate receiver operating characteristic curves and logistic regression analyses. The primary endpoint was an unfunctional outcome at 90 days, defined as a modified Rankin Scale Score of 3-6. Results: We reviewed a total of 247 patients, and the last 134 were included in the study, of whom 41.8% had stroke onset within 6 hours. NWU was significantly lower in patients with stroke onset within 6 hours (6.57±3.43) compared to 6-24 hours (11.69±3.01). Of patients with onset times of 6-24h, the area under the curve (AUC) for distinguishing patient groups according to NWU% was 0.863, with a cut-off value of 9.3 (sensitivity, 80.8%; specificity, 82.1%). A multivariable predictive model including NWU% and age yielded the highest diagnostic ability, with an AUC of 0.857 (sensitivity, 66.7%; specificity, 92.9%). Conclusion: NWU as an imaging biomarker of brain edema predicts functional prognosis after endovascular recanalization therapy in ischemic stroke patients within an extended onset time window.© The author(s).