Syringomyelia is often associated with Chiari malformation, trauma, infection, and spinal cord tumor. Although they are relatively rare diseases, arachnoid cysts and its related pathology, "arachnoid web" can sometimes lead to syrinx formation at the thoracic vertebral level. However, syrinx formation caused by degenerative spinal disorders, particularly at the thoracic vertebral levels, has rarely been reported. Herein, we present a case of syringomyelia with thoracic ossified yellow ligament (OYL) in a 79-year-old man, who underwent initial posterior decompression followed by arachnoid web removal. Posterior decompression via laminectomy of thoracic vertebra 2 and removal of the OYL improved the syrinx partially, but dorsal indentation of the spinal cord and a remnant syrinx were observed in post-operative magnetic resonance images, subsequent to the second surgery's successful removal of the arachnoid web, which had dorsally compressed the spinal cord. After the second operation, the syrinx shrunk further, and the patient could walk independently at 5 months after the operations. In our case, both the OYL and arachnoid web were responsible for syrinx formation. Therefore, the coincidence of degenerative vertebral diseases with a syrinx might indicate the coexistence of an underlying lesion. Furthermore, the arachnoid web in this case might have formed due to the denaturation of the arachnoid cyst triggered by the OYL.