Hydroxycarboxylic acid receptor 3 (HCA<sub>3</sub>), also known as niacin receptor 2 (NIACR2) and GPR109B, is a protein which in humans is encoded by the HCAR3 gene. HCA<sub>3</sub>, like the other hydroxycarboxylic acid receptors HCA<sub>1</sub> and HCA<sub>2</sub>, is a G protein-coupled receptor (GPCR). The primary endogenous agonists of HCA<sub>3</sub> are 3-hydroxyoctanoic acid and kynurenic acid. HCA<sub>3</sub> is also a low-affinity biomolecular target for niacin (aka nicotinic acid).