We report a rare case presenting radiculopathy caused by unilateral lumbosacral facet abnormality resembling facet interlocking. The patient was a 57-year-old man with no medical and traumatic history. He began to exhibit numbness below his left ankle followed by pain at the left buttock with no obvious causes. He visited our hospital approximately 1 year after the onset of his symptom. Preoperative images revealed a left lumbosacral facet abnormality resembling facet interlocking. His left S1 nerve root was compressed by the dislocated left L5 inferior articular process and bone fragment. His symptom was consistent with left S1 radiculopathy without an obvious stenosis of the left L5 intervertebral foramen; thus, we performed partial facetectomy of the left L5/S1, posterior decompression of the S1 nerve root tunnel, and removal of bone fragment. After the operation, his symptom completely disappeared with satisfactory result. There are several types of congenital facet anomalies in the lumbosacral facet joint; however, congenital unilateral lumbosacral facet abnormality resembling facet interlocking described in this paper has not been reported. His clinical symptom was completely recovered after simple decompression surgery. In this paper, we report the interesting and unique findings of facet abnormality resembling facet interlocking.