Arachnoid cysts are rare lesions of the spine and can present with myelopathy, radiculopathy, local pain or a combination of these symptoms. Nerve root prolapse into an arachnoid cyst causing radiculopathy has not been reported before. We report a nerve root prolapse into a spinal arachnoid cyst presenting clinically as radiculopathy. An 18-year-old female patient presented with mid-back pain, right anterior thigh pain and hip flexor weakness. Magnetic resonance imaging (MRI) and computerized tomography (CT) myelography revealed an arachnoid cyst at T12-L1 level on the right side. At surgery, a nerve root was seen prolapsing into an extradural arachnoid cyst. The nerve root was replaced back into dural sac and the dural defect closed. At 20 months of follow-up, the patient continues to be asymptomatic with no evidence of recurrence on imaging. Replacing the prolapsed nerve root into the dural sac with meticulous closure of the dural defect could lead to good clinical outcome. We propose a modification to the popular classification of these lesions to better rationalize their surgical management. Classification of extradural arachnoid spinal cysts (Nabors's type 1) should be based on the presence or absence of dural communication. Sacral meningoceles (Nabors' type 1B) should be excluded from the classification as they have free communication with the thecal sac and are not true spinal cysts.