This report presents a case of a patient with a large synovial cyst treated using a biportal endoscopic ipsilateral approach. Synovial cysts are dilatations of synovial sheaths arising from the zygapophyseal joint capsule. Surgical resection of the cyst and decompression of the neural structures remain the preferred treatment. The size of a cyst is directly proportional to the strength of its attachment to the capsule and dura; thus, larger cysts exhibit a broader area of adhesion between the cyst wall and the dura. The contralateral endoscopic approach has gained popularity for small cysts due to its ability to preserve the facet joint and facilitate cyst dissection. However, larger cysts, which adhere more firmly to the dura, pose greater technical challenges for dissection via the contralateral route. We describe a case of a large synovial cyst successfully treated with a biportal endoscopic ipsilateral approach. A 62-year-old man presented with left leg radicular pain (L5 root) and severe neurogenic intermittent claudication. Preoperative magnetic resonance imaging revealed a large left L4–5 synovial cyst causing severe central canal stenosis. Complete resection was achieved using the ipsilateral unilateral biportal endoscopic technique. Postoperative magnetic resonance imaging confirmed adequate decompression without complications. The patient’s symptoms resolved immediately after surgery, and no spinal instability was observed 1 year postoperatively. The endoscopic biportal ipsilateral approach represents a viable and effective option for the treatment of synovial cysts, particularly those of large size.