The thorax (pl.: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen.

In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the body, each in turn composed of multiple segments.

The human thorax includes the thoracic cavity and the thoracic wall. It contains organs including the heart, lungs, and thymus gland, as well as muscles and various other internal structures. The chest may be affected by many diseases, of which the most common symptom is chest pain.

The word thorax comes from the Greek θώραξ thṓrax "breastplate, cuirass, corslet" via Latin: thorax.

In humans and other hominids, the thorax is the chest region of the body between the neck and the abdomen, along with its internal organs and other contents. It is mostly protected and supported by the rib cage, spine, and shoulder girdle.

The contents of the thorax include the heart and lungs (and the thymus gland); the major and minor pectoral muscles, trapezius muscles, and neck muscle; and internal structures such as the diaphragm, the esophagus, the trachea, and a part of the sternum known as the xiphoid process. Arteries and veins are also contained – (aorta, superior vena cava, inferior vena cava and the pulmonary artery); bones (the shoulder socket containing the upper part of the humerus, the scapula, sternum, thoracic portion of the spine, collarbone, and the rib cage and floating ribs).

External structures are the skin and nipples.

In the human body, the region of the thorax between the neck and diaphragm in the front of the body is called the chest. The corresponding area in an animal can also be referred to as the chest.