The triceps brachii muscle is a large muscle on the back of the upper limb of many vertebrates. It is the muscle principally responsible for extension of the elbow joint (straightening of the arm).

The triceps is an extensor muscle of the elbow joint and an antagonist of the biceps and brachialis muscles. It can also fixate the elbow joint when the forearm and hand are used for fine movements, e.g., when writing. It has been suggested that the long head fascicle is employed when sustained force generation is demanded, or when there is a need for a synergistic control of the shoulder and elbow or both. The lateral head is used for movements requiring occasional high-intensity force, while the medial fascicle enables more precise, low-force movements.

With its origin on the scapula, the long head also acts on the shoulder joint and is also involved in retroversion and adduction of the arm.