The longissimus muscle is one of the three groups of muscles which make up the erector spinae lying either side of the spinous processes of the vertebral column, and forming the largest group of intrinsic back muscles. The iliocostalis is the outermost erector spinae muscle, the longissimus lies in the middle, and the spinalis lies most medially.

Each of the three erector spinae muscles originates from a broad tendon which is attached to the sacrum, spinous processes of the lower thoracic and lumbar vertebrae (T11-L5), thoracolumbar fascia and the iliac crest. In the upper lumbar region, each of the iliocostalis, longissimus and spinalis divide into three columns of muscle, which are then further subdivided into three components. The longissimus divides into the longissimus thoracis, longissimus cervicis and the longissimus capitis.

The erector spinae are covered by the thoracolumbar fascia, lying deep to the rhomboid, splenius and serratus posterior inferior muscles.