The triceps brachii is a major muscle of the upper arm in the human body. The triceps run along the humerus (the main bone of the upper arm) between the shoulder and the elbow. Along with the biceps, it enables extension and retraction of the forearm.

When the triceps are contracted, the forearm extends and the elbow straightens; if the triceps are relaxed and the biceps flexed, the forearm retracts and the elbow bends. The triceps also serve to stabilize the shoulder joint at the top of the humerus. The shoulder has the greatest range of motion of any joint in the body, possessing the ability to turn and rotate in many directions.

However, this movability means that the shoulder is a relatively unstable joint, and the triceps plays an important role in stabilizing it. The distal portions of the triceps are also involved in retracting the capsule of the elbow joint when the forearm extends, thereby allowing the elbow joint to be straightened successfully.

The muscle is supplied with oxygen and nutrients by branches of the deep brachial artery. It is also innervated by four branches of the radial nerve. Both the artery and the nerve continue into the lower forearm beyond the triceps.