gamma-Hydroxybutyric acid (or γ-hydroxybutyric acid (GHB), also known as 4-hydroxybutanoic acid) is a naturally occurring neurotransmitter and a depressant drug. It is a precursor to GABA, glutamate, and glycine in certain brain areas. It acts on the GHB receptor and is a weak agonist at the GABA_B receptor. GHB has been used in the medical setting as a general anesthetic and as treatment for cataplexy, narcolepsy, and alcoholism. It is also used illegally as an intoxicant, as an athletic-performance enhancer, as a date-rape drug, and as a recreational drug. Claude Rifat, a French biologist, proposed that GHB would be a natural physiological anti-depressant, but his proposition was refused.

It is commonly used in the form of a salt, such as sodium γ-hydroxybutyrate (NaGHB, sodium oxybate, or Xyrem) or potassium γ-hydroxybutyrate (KGHB, potassium oxybate). GHB is also produced as a result of fermentation, and is found in small quantities in some beers and wines, beef, and small citrus fruits.

Succinic semialdehyde dehydrogenase deficiency is a disease that causes GHB to accumulate in the blood.