We document a giant spinal extradural arachnoid cyst treated by selective closure of the dural defect. It was diagnosed using cinematic magnetic resonance imaging (cine-MRI). To demonstrate the effectiveness of selective closure of the dural defect as a treatment for a spinal extradural arachnoid cyst. The standard treatment for a spinal extradural arachnoid cyst is total resection of the cyst wall, if possible, and the closure of the communication site, if any, between the cyst and the subarachnoid space, after an extensive laminectomy. To our knowledge, selective closure of the dural defect through minimal laminotomy with little cyst resection has not been reported. A 29-year-old woman presented with right leg muscle weakness and was diagnosed with an extradural arachnoid cyst ranging from spinal regions T11 to L3 using MRI. Myelography demonstrated that the cyst communicated with the subarachnoid space. Cine-MRI showed a pulsating flow voiding on the left side of level L1, suggesting the location of the communication site. Fenestration of the T12-L1 region was performed, preserving the spinous processes and the facet joints. A small dural rent was found on the left side of level L1. This was closed using small clips. MRI 12 days later demonstrated that the cyst had shrunk dramatically. It had disappeared completely by 4 months. The patient's muscle weakness improved gradually, and she was almost complaint-free 6 months after the operation. Selective closure of the dural defect based on cine-MRI will be useful for treating extradural arachnoid cysts.