Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are:

At birth, children may be asymptomatic or present with many severe symptoms. Later in infancy, there are typically episodes of bluish colour to the skin due to a lack of sufficient oxygenation, known as cyanosis. When affected babies cry or have a bowel movement, they may undergo a "tet spell" where they turn cyanotic, have difficulty breathing, become limp, and occasionally lose consciousness. Other symptoms may include a heart murmur, finger clubbing, and easy tiring upon breastfeeding.

The cause of tetralogy of Fallot is typically not known. Maternal risk factors include lifestyle-related habits (alcohol use during pregnancy, smoking, or recreational drugs), medical conditions (diabetes), infections during pregnancy (rubella), and advanced age of mother during pregnancy (35 years and older). Babies with Down syndrome and other chromosomal defects that cause congenital heart defects may also be at risk of tetralogy of Fallot.

Tetralogy of Fallot is typically treated by open heart surgery in the first year of life. The timing of surgery depends on the baby's symptoms and size. The procedure involves increasing the size of the pulmonary valve and pulmonary arteries and repairing the ventricular septal defect. In babies who are too small, a temporary surgery may be done with plans for a second surgery when the baby is bigger. With proper care, most people who are affected live to be adults. Long-term problems may include an irregular heart rate and pulmonary regurgitation.

The prevalence is estimated to be anywhere from 0.02 to 0.04% in the general population. Though males and females were initially thought to be affected equally, more recent studies have found males to be affected more than females. It is the most common complex congenital heart defect, accounting for about 10 percent of cases. It was initially described in 1671 by Niels Steensen. A further description was published in 1888 by the French physician Étienne-Louis Arthur Fallot, after whom it is named. The first total surgical repair was carried out in 1954.

Tetralogy of Fallot results in low oxygenation of blood. This is due to a mixing of oxygenated and deoxygenated blood in the left ventricle via the ventricular septal defect (VSD) and preferential flow of the mixed blood from both ventricles through the aorta because of the obstruction to flow through the pulmonary valve. The latter is known as a right-to-left shunt.

Infants with TOF – a cyanotic heart disease – have low blood oxygen saturation. Blood oxygenation varies greatly from one patient to another depending on the severity of the anatomic defects. Typical ranges vary from 60% to around 90%. Depending on the degree of obstruction, symptoms vary from no cyanosis or mild cyanosis to profound cyanosis at birth. If the baby is not cyanotic, then it is sometimes referred to as a "pink tet". Other symptoms include a heart murmur which may range from almost imperceptible to very loud, difficulty in feeding, failure to gain weight, retarded growth and physical development, labored breathing (dyspnea) on exertion, clubbing of the fingers and toes, and polycythemia. The baby may turn blue with breastfeeding or crying.

Those born with tetralogy of Fallot are more likely to experience psychiatric disorders such as attention deficit hyperactivity disorder (ADHD) in later life, potentially due to underlying genetic changes that predispose to both conditions.