Aberrant muscle bundles of approximately 10 cm in length, running subcutaneously from the posterior surface of sacrum to the iliac crest, were found on both sides of three males among 93 cadavers of anatomical courses for medical students. Since no precedent of this anomaly has been described in the literature, we present its morphology and discuss the significance of this muscle. This muscle originated from the tendinous fibers of the superficial-most layer of the sacral part of thoracolumbar fascia, became muscularized near the posterior superior iliac spine, passed over the origins of the gluteus maximus and medius, and inserted on the iliac crest. Some bundles crossed the iliac crest to insert between sublayers of the posterior layer of the thoracolumbar fascia. The superior cluneal nerves passed above, below, or penetrated the muscle. Branches from the segmentally lowest one or the conjoined lowest two entered the aberrant muscles. Considering its location and multi-layered structure, the muscle is likely to be a muscularized bundle of the thoracolumbar fascia. The aberrant muscle is also similar to the serratus posterior inferior in its location at the lateral end of the thoracolumbar fascia and below the latissimus dorsi. Nevertheless its innervation from the posterior rami of the spinal nerves and partial fusion at its origin implies a close relationship to the iliocostalis. This aberrant muscle may also be of clinical interest since it is potentially involved in the entrapment of the superior cluneal nerves as it runs along the caudolateral edge of the thoracolumbar fascia.