BACKGROUND AND PURPOSE: Evidence for intra-arterial thrombolysis (IAT) after endovascular treatment (EVT) remains limited in acute basilar artery occlusion (ABAO), particularly among patients with successful but incomplete reperfusion after EVT. We aimed to evaluate the effectiveness and safety of adjunct IAT following EVT. METHODS: This study was derived from the BASILAR registry. Patients who were treated with and without intra-arterial thrombolysis after successful but incomplete recanalization with a modified thrombolysis in cerebral infarction (mTICI) score of 2b were included. Enrolled patients were divided into EVT plus IAT group and EVT alone group for comparison. The primary effectiveness outcome was the distribution of modified Rankin Scale (mRS) score at 90 days. Safety outcomes included 90-day mortality and symptomatic intracranial hemorrhage within 48 h. RESULTS: 211 patients were enrolled, 43 (20.4%) received EVT plus IAT. EVT plus IAT was associated with a favorable shift in 90-day mRS distribution [unadjusted common odds ratio (OR) 1.95, 95% confidence interval (CI) 1.00-3.76; P = 0.048; adjusted common OR 2.59, 95% CI 1.15-5.79; P = 0.022]. For safety outcomes, EVT plus IAT was associated with lower 90-day mortality (adjusted OR 0.35, 95% CI 0.14-0.88, P = 0.026), while symptomatic intracranial hemorrhage (sICH) did not differ significantly between groups (P = 0.356). In addition, exploratory analysis demonstrated that baseline stroke severity did not modify the treatment effect of IAT. CONCLUSIONS: In patients with ABAO who achieved successful but incomplete reperfusion after EVT, adjunct IAT was associated with improved functional outcome and lower mortality and may not increase of sICH.