The semispinalis capitis is a long, thick muscle that underlies the trapezius and the splenius; but, in thin people, it produces the rounded muscular ridge seen at the side of the median furrow of the back of the neck. It has the same origin as the longissimus capitis, and is inserted into the medial half of the area between the superior and inferior nuchal lines. It is separated from its fellow muscle of the opposite side by a membranous partition called the ligamentum nuchae.

It bends the head backwards; and is supplied by the posterior primary rami of the upper cervical nerves.