Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic disorder in which a female is partially or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hairline at the back of the neck, short stature, and swollen hands and feet are seen at birth. Typically, those affected do not develop menstrual periods, or breasts without hormone treatment and are unable to have children without reproductive technology. Heart defects, diabetes, and low thyroid hormone occur in the disorder more frequently than average. Most people with Turner syndrome have normal intelligence; however, many have problems with spatial visualization that may be needed in order to learn mathematics. Vision and hearing problems also occur more often than average.

Turner syndrome is not usually inherited; rather, it occurs during formation of the reproductive cells in a parent or in early cell division during development. No environmental risks are known, and the mother's age does not play a role. While most people have 46 chromosomes, people with Turner syndrome usually have 45 in some or all cells. The chromosomal abnormality is often present in just some cells, in which case it is known as Turner syndrome with mosaicism. In these cases the symptoms are usually fewer, and possibly none occur at all. Diagnosis is based on physical signs and genetic testing.

No cure for Turner syndrome is known. Treatment may help with symptoms. Human growth hormone injections during childhood may increase adult height. Estrogen replacement therapy can promote development of the breasts and hips. Medical care is often required to manage other health problems with which Turner syndrome is associated.

Turner syndrome occurs in between one in 2,000 and one in 5,000 females at birth. All regions of the world and cultures are affected about equally. Generally people with Turner syndrome have a shorter life expectancy, mostly due to heart problems and diabetes. American endocrinologist Henry Turner first described the condition in 1938. In 1964, it was determined to be due to a chromosomal abnormality.