The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. It is classified as the eleventh of twelve pairs of cranial nerves because part of it was formerly believed to originate in the brain. The sternocleidomastoid muscle tilts and rotates the head, whereas the trapezius muscle, connecting to the scapula, acts to shrug the shoulder.

Traditional descriptions of the accessory nerve divide it into a spinal part and a cranial part. The cranial component rapidly joins the vagus nerve, and there is ongoing debate about whether the cranial part should be considered part of the accessory nerve proper. Consequently, the term "accessory nerve" usually refers only to nerve supplying the sternocleidomastoid and trapezius muscles, also called the spinal accessory nerve.

Strength testing of these muscles can be measured during a neurological examination to assess function of the spinal accessory nerve. Poor strength or limited movement are suggestive of damage, which can result from a variety of causes. Injury to the spinal accessory nerve is most commonly caused by medical procedures that involve the head and neck. Injury can cause wasting of the shoulder muscles, winging of the scapula, and weakness of shoulder abduction and external rotation.

The accessory nerve is derived from the basal plate of the embryonic spinal segments C1–C6.

The fibres of the spinal accessory nerve originate solely in neurons situated in the upper spinal cord, from where the spinal cord begins at the junction with the medulla oblongata, to the level of about C6. These fibres join to form rootlets, roots, and finally the spinal accessory nerve itself. The formed nerve enters the skull through the foramen magnum, the large opening at the skull's base. The nerve travels along the inner wall of the skull towards the jugular foramen. Leaving the skull, the nerve travels through the jugular foramen with the glossopharyngeal and vagus nerves. The spinal accessory nerve is notable for being the only cranial nerve to both enter and exit the skull. This is due to it being unique among the cranial nerves in having neurons in the spinal cord.

After leaving the skull, the cranial component detaches from the spinal component. The spinal accessory nerve continues alone and heads backwards and downwards. In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of digastric muscle. As it courses downwards, the nerve pierces through the sternocleidomastoid muscle (approximately 1 cm above Erb's point) while sending it motor branches, then continues down until it reaches the trapezius muscle (entering at the junction of the middle and lower third of the anterior border of the trapezius) to provide motor innervation to its upper part.

The fibres that form the spinal accessory nerve are formed by lower motor neurons located in the upper segments of the spinal cord. This cluster of neurons, called the spinal accessory nucleus, is located in the lateral aspect of the anterior horn of the spinal cord, and stretches from where the spinal cord begins (at the junction with the medulla) through to the level of about C6. The lateral horn of high cervical segments appears to be continuous with the nucleus ambiguus of the medulla oblongata, from which the cranial component of the accessory nerve is derived.

In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of digastric muscle, in front of the vein in about 80% of people, and behind it in about 20%, and in one reported case, piercing the vein.