A 45-year-old woman presented with complaints of low back pain and sciatica on the left persisting for 2 years. She had undergone left hemilaminectomy and discectomy for L4-5 intervertebral disc herniation at another medical center. Spinal computed tomography and magnetic resonance (MR) imaging revealed a mass lesion in the posterior paravertebral region. The mass was hypointense with ring enhancement on the T₁-weighted images and hyperintense on the T₂-weighted images. Surgery found a retained sponge within the paraspinal mass cavity which was removed totally. Foreign-body granuloma (“gauzoma”) induced by forgotten sponge material is not an unusual complication of posterior lumbar surgery and should be considered as a potential cause in cases of surgical wound infections. MR imaging is essential to achieve the correct differential diagnosis.