Background: With the expanding eligibility for endovascular therapy (EVT) of patients presenting in the late window (6-24 hours after last known well), we aimed to derive a score to predict favorable outcomes associated with EVT versus best medical management (BMM). Such a scale could be used to identify patients likely or unlikely to benefit from EVT in the late window. Methods: The multinational Computed Tomography for Late Endovascular Reperfusion (CLEAR) observational cohort of consecutive patients with acute proximal intracranial occlusion (2014-2022) was queried (n=58 sites). Logistic regression analyses were used to derive an 8-point score for predicting good functional outcome (mRS 0-2 or return to premorbid mRS) at 90 days, with sensitivity analyses for pre-specified subgroups conducted using bootstrapped random forest regressions and was externally validated with a single center cohort (2014-2023). Secondary outcomes included 90-day functional independence (mRS 0-2), symptomatic intracranial hemorrhage (sICH), poor outcome (mRS 5-6), and 90-day survival. Findings: Of the 3231 included patients (n=2499 treated with EVT), an 8-point CLEAR score included age, ASPECTS, and National Institutes of Health Stroke Scale (NIHSS), with higher points indicating a higher probability of a good functional outcome. The areas under the curve (AUC) for the primary outcome among EVT and BMM subgroups were 0·72 (95% confidence interval [CI]: 0·70-0·74) and 0·87 (95% CI: 0·84-0·90), respectively, with similar performance in the external validation cohort (AUC 0·71, 95% CI: 0·66-0·76). There was a significant interaction between CLEAR score and EVT treatment for good functional outcome, functional independence, and poor outcome (all pinteraction <0·001), with greater benefit favoring patients with lower and mid-range CLEAR scores. Interpretation: The CLEAR Thrombectomy Success Score is a pragmatic tool that can estimate the probability of a good outcome in patients treated with EVT versus BMM in the late window for proximal anterior circulation occlusion. Funding: Medtronic.