Tethered cord syndrome (TCS) is a clinical diagnosis of progressive neurological aggravation of the lower spinal cord due to a traction on the conus medullaris (CM). Untethering surgery is effective for most TCS, however, when anatomical variations of spinal cord and filum terminale (FT) exist, regular untethering may lead to a failed outcome.The authors present the case of a 45-year-old patient with TCS caused by duplicated FT with split cord malformation (SCM). Lumbosacral magnetic resonance imaging (MRI) revealed a type II SCM with a significant low-lying CM. Laminectomy was performed. Neurophysiological monitoring was used for nerve root identification and two thickened fila, which failed to respond on stimulation, were found during the surgery. Both fila were sectioned and the diagnosis was finally confirmed by pathological examination. Postoperatively, the patient's symptoms disappeared immediately and no neurological sequela was found after surgery.This is the first documented adult of duplicated FT with preoperative radiological evidence, and reported in association with SCM as a cause of TCS. When SCM exists, a careful observation for duplicated FT is warranted on preoperative MRI and during surgery. Complete transection of the double FT under intraoperative neurophysiological monitoring is the best treatment for this anomaly.