OBJECTIVE First-pass effect (FPE), defined as successful reperfusion (modified Thrombolysis in Cerebral Infarction score 2b–3) with a single stent retriever attempt without salvage treatment, has not been fully identified in patients with acute basilar artery occlusion (BAO). The authors’ aim was to assess the impact of FPE on efficacy and safety for patients with BAO. METHODS The authors included data from the Acute Basilar Artery Occlusion Study (BASILAR) about patients who underwent mechanical thrombectomy within 24 hours after symptom onset and compared the clinical outcomes of patients who achieved FPE with those who did not. In addition, the authors further compared outcomes between patients with FPE and those with final successful reperfusion achieved with salvage treatment. The primary clinical outcome was favorable outcome (modified Rankin Scale score ≤ 3). RESULTS Among 471 enrolled patients, FPE was achieved in 83 (17.6%) who underwent acute BAO thrombectomy. FPE was strongly associated with favorable outcome (adjusted OR 2.84, 95% CI 1.56–5.16, p = 0.001), lower rate of mortality (28.9% of FPE patients vs 48.2% of non-FPE patients, p = 0.001), and shorter median time from groin puncture to recanalization (65 minutes vs 110 minutes, p < 0.001). Occlusion site of the distal basilar artery, cardioembolism, and undetermined etiology were positive predictors of FPE, whereas baseline National Institutes of Health Stroke Scale score was a negative predictor. Compared with final successful reperfusion, FPE also contributed independently to favorable outcomes (adjusted OR 2.25, 95% CI 1.23–4.10, p = 0.008). CONCLUSIONS FPE was associated with 90-day favorable outcome in patients with acute BAO who underwent stent retriever thrombectomy within 24 hours. Clinical trial registration no.: ChiCTR1800014759 (www.chictr.org.cn)