Summary: Background: Brain metastases (BMs) in non-small cell lung cancer (NSCLC) are associated with poor prognosis and quality of life (QoL). This study aimed to evaluate the efficacy and safety of online adaptive MR-guided fractionated stereotactic radiotherapy (FSRT) using a 1.5 T MR-Linac in this subgroup of patients. Methods: This single-arm phase 2 trial was conducted at Sun Yat-sen University Cancer Centre. Patients aged 18–75 years with NSCLC, 1–10 BMs, and an ECOG status of 0–1 were included. Key exclusion criteria included inability to undergo contrast-enhanced MRI and contraindications to bevacizumab. Patients received 30 Gy adaptive FSRT in 5 daily fractions under real-time MR guidance, with bevacizumab before (day 1) and after (day 21) FSRT. The primary endpoint was 1-year intracranial progression-free survival (IPFS); secondary endpoints included objective response rate (ORR), 1-year progression-free survival (PFS), 1-year overall survival (OS), treatment-related toxicities, and QoL. All enrolled patients were included in primary and safety analyses. This trial is registered with Clinicaltrials.gov, NCT04946019. Findings: Between June 10th, 2021 and June 29th, 2023, 70 patients were assessed for eligibility and 55 patients were enrolled (median follow-up: 22.3 months). The median age was 58 years (IQR: 51–65), with 33% (18/55) female patients, and 82% (45/55) presenting with adenocarcinoma. The 1-year IPFS rate was 78.7% (95% CI, 68.2%–90.7%), with a median IPFS of 21.9 months (95% CI, 13.8–30.1 months). The 1-year PFS rate was 63.5% (95% CI: 51.8%–78.2%), and OS was 82.4% (95% CI: 72.6%–93.6%). The ORR reached 78% (95% CI: 65.0%–88.2%). Treatment-related toxicity was minimal, with only one case (2%) of grade 1 radiation necrosis. QoL improved steadily, with the Global Health Status score increasing from 65.67 ± 16.97 to 79.33 ± 8.79 at 6 months post FSRT (p