We present a case that had two separate facet joints on the same side causing an intervertebral instability. The embryological pathogenesis of the congenital existence of two separate facet joints on the same side of the vertebra is not conclusively known. A 68-year-old woman presented with lower back pain and severe left leg pain. Neuroradiological evaluation including dynamic plain radiography, computed tomography, and magnetic resonance imaging of the lumbar spine revealed the existence of two separate facet joints on the same side of the first sacral vertebra, severe degenerative changes of both right and left L5-S1 facet joints, and Grade II L5-S1 spondylolisthesis. Subsequently, she underwent surgery. Intraoperatively, two separate facet joints on the same side of the first sacral vertebra were confirmed. The patient's symptoms were resolved after decompression and fusion surgery. This is a unique case of the congenital existence of two separate facet joints on the same side of the first sacral vertebra.