Thoracic outlet syndrome (TOS) is a condition in which there is compression of the nerves, arteries, or veins in the superior thoracic aperture, the passageway from the lower neck to the armpit, also known as the thoracic outlet. There are three main types: neurogenic, venous, and arterial. The neurogenic type is the most common and presents with pain, weakness, paraesthesia, and occasionally loss of muscle at the base of the thumb. The venous type results in swelling, pain, and possibly a bluish coloration of the arm. The arterial type results in pain, coldness, and pallor of the arm.

TOS may result from trauma, repetitive arm movements, tumors, pregnancy, or anatomical variations such as a cervical rib. The diagnosis may be supported by nerve conduction studies and medical imaging. TOS is difficult to diagnose and there are many potential differential diagnoses as well as other diseases that are often co-occurrent with TOS.

Initial treatment for the neurogenic type is with exercises to strengthen the chest muscles and improve posture. NSAIDs such as naproxen may be used for pain. Surgery is typically done for the arterial and venous types and for the neurogenic type if it does not improve with other treatments. Blood thinners may be used to treat or prevent blood clots. The condition affects about 1% of the population. It is more common in women than men and it occurs most commonly between 20 and 50 years of age. The condition was first described in 1818 and the current term "thoracic outlet syndrome" first used in 1956.