Gossypiboma is a mass formed by a retained surgical sponge and reactive tissue. The cases with gossypiboma are usually asymptomatic or with nonspecific symptoms, which delay diagnosis for months or years after surgery. We describe imaging findings in a 43-year-old woman with a symptomatic retained surgical sponge in a lumbar laminectomy site. Ultrasonography, computed tomography, magnetic resonance imaging (MRI), and diffusion-weighted MRI were performed. Gossypiboma should be considered in the differential diagnosis of a mass in a patient with a history of prior surgery. Diffusion-weighted MRI may provide important data for differential diagnosis of gossypiboma. With diffusion-weighted MRI, gossypiboma may be distinguished from an abscess by its low signal intensity and increased apparent diffusion coefficient (ADC) as compared to high signal intensity with low ADC in cases of abscess.