The ulnar nerve is a nerve that runs near the ulna, one of the two long bones in the forearm. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest in the human body unprotected by muscle or bone, so injury is common. This nerve is directly connected to the little finger, and the adjacent half of the ring finger, innervating the palmar aspect of these fingers, including both front and back of the tips, perhaps as far back as the fingernail beds.

This nerve can cause an electric shock-like sensation by striking the medial epicondyle of the humerus posteriorly, or inferiorly with the elbow flexed. The ulnar nerve is trapped between the bone and the overlying skin at this point. This is commonly referred to as bumping one's "funny bone". This name is thought to be a pun, based on the sound resemblance between the name of the bone of the upper arm, the humerus, and the word "humorous". Alternatively, according to the Oxford English Dictionary, it may refer to "the peculiar sensation experienced when it is struck".

The ulnar nerve originates from the C8-T1 nerve roots (and occasionally carries C7 fibers which arise from the lateral cord), which then form part of the medial cord of the brachial plexus, and descends medial to the brachial artery, up until the insertion point of coracobrachialis muscle (middle 5 cm over the medial border of the humerus). Then, it pierces the medial intermuscular septum and enters the posterior compartment of the arm, accompanied by superior ulnar collateral vessels. It runs at the posteromedial aspects of the humerus, passing behind the medial epicondyle (in the cubital tunnel) at the elbow, where it can be palpated.

It enters the anterior (flexor) compartment of the forearm between the two heads of flexor carpi ulnaris, and lies along the lateral border of the flexor carpi ulnaris. The ulnar nerve runs between the flexor digitorum superficialis (laterally) and flexor digitorum profundus medially. Near the wrist, it courses superficial to the flexor retinaculum of hand, but covered by volar carpal ligament to enter the hand.

In the forearm it gives off the following branches:

Ulnar nerve enters the palm of the hand via the Guyon's canal, superficial to the flexor retinaculum and lateral to the pisiform bone.

Here it gives off the following branches:

The ulnar nerve is also known as the "musician's nerve" as it controls the fine movements of the fingers.