Background <p>Fascial tissues have been overlooked in the past, and there are only few studies on the thoracolumbar fascia and its treatment methods. The present study aimed to investigate the effectiveness and safety of ultrasound-guided bilateral L3-4 thoracolumbar fascia injection in treating postoperative low back pain.</p> Methods <p>A prospective pilot study was conducted at a university hospital. Twenty-six patients with postoperative low back pain resistant to conservative therapies and US-guided bilateral L3-4 medial branch block (MBB) were included as participants. They were treated with US-guided L3-4 mid-thoracolumbar fascia injection. Using real-time fluoroscopy, we monitored the contrast medium’s diffusion range and double-checked the needle tips’ positions throughout the procedures. The pain numeric rating scales (NRS), lumbar anteflexion range of motion (ROM), and Oswestry disability index (ODI) were assessed through telephone interviews one, four, and twelve weeks after the procedures. All data were processed by SPSS software version 23.0 (IBM Corp., New York).</p> Results <p>Compared with NRS at baseline, pain scores decreased throughout the observation period. Lumbar anteflexion range of motion of US-guided bilateral L3-4 mid-thoracolumbar fascia injection continuously improved during the first, fourth, and twelfth week. No intravascular injections or neurologic injuries were observed.</p> Conclusion <p>The US-guided bilateral L3-4 mid-thoracolumbar fascia injection facilitated a uniform dispersion of the medication, akin to the gentle spread of a goose feather, between the erector spinalis and quadratus lumborum muscles across the affected lumbar vertebrae. This technique demonstrated substantial clinical effectiveness in patients who were unresponsive to standard US-guided MBB.</p>