Of many reports referring to injury mechanism in anterior lumbosacral dislocation, there were none concerning hyperextension mechanism. We report a case of a 46-year-old man with preexisting L5 spondylolysis sustaining traumatic complete anterior lumbosacral dislocation. The operative findings, together with the radiologic findings, strongly suggested that the dislocation occurred by hyperextension mechanism. Open reduction was done by applying force of distraction with flexion using a rod and screw system, followed by the internal fixation from the L3 to S1 vertebrae and the postero-superior iliac spine. The lumbosacral dislocation was reduced to 77%. At the follow-up at 5 years after surgery, bony union was obtained and the patient could move with a wheelchair although the neurologic deficit in lower extremities observed preoperatively did not recover. Preexisting L5 spondylolysis was considered to increase the potential for anterior lumbosacral dislocation by additional force of compression with hyperextension. Posterior instrumentation using a rod and screw system was considered a useful method for reduction, decompression, stabilization, and fusion.