Purpose <p>This study aimed to assess paraspinal muscle changes following biportal endoscopic and microscopic lumbar spine surgery, including both discectomy and decompression procedures.</p> Methods <p>A total of 156 patients who underwent single-segment lumbar discectomy or posterior decompression were analysed (biportal endoscopic spine surgery [BESS] group: 86; Micro group: 70). Magnetic resonance imaging (MRI) was used to evaluate paraspinal muscle volume (cross-sectional area) and degeneration (fatty infiltration ratio) preoperatively and at 1 year postoperatively. Both the multifidus and total paraspinal muscles were assessed on the operative and contralateral sides. Clinical outcomes—including postoperative pain, functional disability, and health-related quality of life—were also compared between the two groups.</p> Results <p>Preoperative paraspinal muscle conditions were comparable between the groups. At 1 year postoperatively, the BESS group showed no significant muscle changes from baseline or between sides. In contrast, the Micro group demonstrated a significant increase in multifidus muscle fatty infiltration on the ipsilateral side of the approach compared to the contralateral side. The increase was also significantly greater than that observed in the BESS group. Changes in the total paraspinal muscle were not significantly different between the groups. Clinically, both groups showed good outcomes; however, the BESS group reported a small but significant advantage in pain reduction.</p> Conclusion <p>Biportal endoscopic surgery, whether for lumbar discectomy or decompression, results in less fatty degeneration of the multifidus muscle compared to microscopic techniques. These findings suggest that BESS may better preserve paraspinal muscle integrity in minimally invasive lumbar surgery.</p>