Galactosemia (British galactosaemia, from Greek γαλακτόζη + αίμα, meaning galactose + blood, accumulation of galactose in blood) is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galactose properly. Galactosemia follows an autosomal recessive mode of inheritance that confers a deficiency in an enzyme responsible for adequate galactose degradation.

Friedrich Goppert (1870–1927), a German physician, first described the disease in 1917, with its cause as a defect in galactose metabolism being identified by a group led by Herman Kalckar in 1956. Galactosemia was the second disorder found to be detectable through newborn screening methods by Robert Guthrie.

Its incidence is about 1 per 60,000 births for people of European ancestry. In other populations the incidence rate differs. Galactosemia is about one hundred times more common (1:480 births) in the Irish Traveller population.

Galactosemia is generally diagnosed in neonates after symptoms are shown or during routine newborn screening. However, there have been rare cases of undiagnosed galactosemia in adults, usually presenting with developmental delay in addition to numerous other symptoms. Adults with galactosemia are at higher risk for cataracts (See: Galactosemic Cataract), with a prevalence of 1 in 5. Adults with galactosemia are at a much greater risk for an anxiety disorder (prevalence of 1 in 2, or 50 percent) and a slightly elevated risk of depression (prevalence of 3 in 25, or 12 percent). They may also have ADHD, tremors, seizures or other neurological or psychological problems.

Infants may appear asymptomatic at birth; however, upon ingestion of galactose a few days later (via breast and/or formula feeding), children start to experience life-threatening symptoms, which include:

Progression of this acute neonatal toxicity syndrome may include the development of sepsis, cataracts, and even pseudotumor cerebri (which may cause a bulging of fontanelle).

In children, untreated galactosemia can lead to cataracts, developmental delays, intellectual disabilities, speech difficulties, fine and gross motor difficulties, kidney disease, liver failure, sepsis, and premature ovarian insufficiency. While a child is being treated for galactosemia, they may continue to experience speech delays, learning disabilities, behavioral issues, ataxia, tremors, and hormone deficiencies.

Lactose in food (such as dairy products) is broken down by the enzyme lactase into glucose and galactose. In individuals with galactosemia, the enzymes needed for further metabolism of galactose (galactokinase and galactose-1-phosphate uridyltransferase) are severely diminished or missing entirely, leading to toxic levels of galactose or galactose 1-phosphate (depending on which enzyme is missing) in various tissues as in the case of classic galactosemia, resulting in hepatomegaly (an enlarged liver), cirrhosis, kidney failure, cataracts, vomiting, seizure, low blood sugar (hypoglycemia), lethargy, brain damage, and ovarian failure. Without treatment, mortality in infants with galactosemia is about 75%.