Niacin, also known as vitamin B3 or nicotinic acid, is a water-soluble vitamin that prevents the deficiency disease pellagra. It is an organic compound with the molecular formula C6H5NO2. It is a derivative of pyridine, with a carboxyl group (COOH) at the 3-position. Other forms of vitamin B3 include the corresponding amide, nicotinamide ("niacinamide"), where the carboxyl group has been replaced by a carboxamide group (CONH2), as well as more complex amides and a variety of esters. The terms niacin, nicotinamide, and vitamin B3 are often used interchangeably to refer to any one of this family of molecules, since they have a common biochemical activity.

Niacin, also known as nicotinic acid or vitamin B3, is a water-soluble vitamin discovered by Conrad Elvehjem in 1937. Its derivatives, NADH, NAD, NAD+, and NADP play essential roles in energy metabolism in the living cell and DNA repair (an enzymatic process in a living cell). The designation vitamin B3 also includes the corresponding amide nicotinamide (or "niacinamide"), whose chemical formula is C6H6N2O. Other functions of niacin include removing toxic chemicals from the body, and assisting in the production of steroid hormones made by the adrenal gland, such as sex hormones and stress-related hormones.