Tay-Sachs disease (abbreviated TSD, also known as GM2 gangliosidosis or Hexosaminidase A deficiency) is an autosomal recessive genetic disorder. In its most common variant known as infantile Tay-Sachs disease it presents with a relentless deterioration of mental and physical abilities which commences at 6 months of age and usually results in death by the age of four.

It is caused by a genetic defect in a single gene with one defective copy of that gene inherited from each parent. The disease occurs when harmful quantities of gangliosides accumulate in the nerve cells of the brain, eventually leading to the premature death of those cells. There is currently no cure or treatment. Tay-Sachs disease is a rare disease. Other autosomal disorders such as cystic fibrosis and sickle cell anemia are far more common.

The disease is named after the British ophthalmologist Warren Tay who first described the red spot on the retina of the eye in 1881, and the American neurologist Bernard Sachs of Mount Sinai Hospital who described the cellular changes of Tay-Sachs and noted an increased prevalence in the Eastern European Jewish (Ashkenazi) population in 1887.

Research in the late 20th century demonstrated that Tay-Sachs disease is caused by a genetic mutation on the HEXA gene on chromosome 15. A large number of HEXA mutations have been discovered, and new ones are still being reported. These mutations reach significant frequencies in several populations. French Canadians of southeastern Quebec have a carrier frequency similar to Ashkenazi Jews, but they carry a different mutation. Many Cajuns of southern Louisiana carry the same mutation that is most common in Ashkenazi Jews. Most HEXA mutations are rare, and do not occur in genetically isolated populations. The disease can potentially occur from the inheritance of two unrelated mutations in the HEXA gene.

TSD is an autosomal recessive genetic disorder, meaning that when both parents are carriers, there is a 25% risk of giving birth to an affected child.