Prior studies on anterior circulation stroke have demonstrated that the benefits of endovascular treatment (EVT) may be absent in patients with poor collaterals. Our study focused on patients with basilar artery occlusion (BAO) to investigate time-dependent EVT effects according to the posterior circulation collateral score (PC-CS). The BASILAR study was a nationwide prospective Chinese registry of consecutive BAO patients. Patients were divided into groups receiving standard medical therapy alone (SMT group) or SMT plus EVT (EVT group). Restricted cubic spline analyses (RCSA) were performed to explore the nonlinear and linear relationships between EVT time and outcomes for different PC-CS. We included 828 patients with acute BAO. Compared with the poor collateral (PC-CS 0-3), the adjusted odds ratio of favorable outcome was 1.311 in patients with moderate (PC-CS 4-5) (95% CI, 0.781-2.201) and 1.899 with good (PC-CS 6-10) collateral (1.125-3.207) for EVT. RCSA revealed that in patients with PC-CS 0-3, the favorable outcome probability after EVT significantly decreased to 10% within 6 h and stabilized thereafter (Pnonlinearity = 0.035), while in patients with moderate and good collateral, the probability was maintained at approximately 30% and 40% respectively, even beyond 6 h (all Pnonlinearity > 0.05). Among patients with BAO, good collateral circulation was independently associated with improved outcomes along with the usage of thrombectomy. Patients with poor collaterals should receive EVT as early as possible, especially within 6 h of symptom onset, while the time window may be extended in patients with moderate and good collaterals. Unique identifier: ChiCTR1800014759.© 2022. The American Society for Experimental Neurotherapeutics, Inc.