In human anatomy, the trapezius is a large superficial muscle on a person's back. It is innervated by the accessory nerve (Cranial Nerve 11) and the ventral rami of cervical vertebrae 3 and 4, which also supplies the sternocleidomastoid muscle.

The two Trapezius muscles together resemble a trapezium, or diamond-shaped quadrangle: two angles corresponding to the shoulders; a third to the occipital protuberance; and the fourth to the spinous process of the twelfth thoracic vertebra

Superficial dissection of the right side of the neck, showing the carotid and subclavian arteries.

The arteries of the face and scalp.

Because the fibers run in different directions, it has a variety of actions, including:

• scapular abduction (shrugging up or lifting the shoulders)
• scapular retraction (drawing the shoulder blades toward the midline)
• scapular depression (pulling the shoulder blades down)

Different fibers control different actions:

• The superior (upper) fibers elevate the scapula
• the middle fibers retract it
• The inferior (lower) fibers depress it
• When the superior and inferior fibers act together, they superiorly (upwardly) rotate the scapula