The lat muscle, or latissimus dorsii is a large, roughly triangular skeletal muscle which stretches from near the spinal column across the back and up under the armpit to join the humerus, the upper arm bone. Each person has two latissimus dorsii muscles, one on each side of the body. The lat muscle is one of a group of muscles responsible for several movements of the arm and shoulder joint.

Muscles of the back are among some of the largest in the human body, and the lat muscle is the largest of these. Its full name, latissimus dorsii means "broadest muscle of the back" in Latin and this muscle is an important part of the complex of muscles that perform a wide variety of movements of the arm and shoulder. Motions governed or aided by the lat muscle include extension, flexion, adduction, and rotation of the shoulder joint. Nearly every movement performed by the shoulder engages the lat muscle to some extent.

The latissimus dorsii also performs secondary functions in regards to movement of the torso and spine. It acts in a synergistic fashion with other muscles of the torso to extend and flex the lower spine. It also serves to help stabilize the torso.

The basic structure of the lat muscle is the same from person to person, but details of its attachment to the spine and interaction with other muscles may vary between individuals. The broad, fan-shaped portion near the spine attaches at various points to the vertebrae; it may attach to as few as four or as many as eight vertebrae. The muscle may or may not attach to the iliac crest. A small extension of the muscle may envelop the tendon which attaches the lat muscle to the humerus.

The lat muscle is commonly targeted during exercise, both in weight training and in low-impact, highly aerobic activities. Classic weight lifting motions like the dead lift and dumbbell row are good for exercising the lat muscles. Some other common exercises that work these muscles are chin-ups, pull-ups standard rows, and pull downs. Many of these exercises also engage other muscles like the trapezius, deltoid, and biceps.

Tightness of the lat muscle can cause pain in many ways. Chronic back pain is the most common. Its connection to the upper arm and shoulder-motion functions can also cause pain in these areas if the lat muscle is overly tight. A good stretching exercise for the lat muscle is to extend both arms upward and slightly behind the head, fingers interlaced, palms facing upward.