Hydroxycarboxylic acid receptor 3 (HCA₃), also known as niacin receptor 2 (NIACR2) and GPR109B, is a protein which in humans is encoded by the HCAR3 gene. HCA₃, like the other hydroxycarboxylic acid receptors HCA₁ and HCA₂, is a G_i/o-coupled G protein-coupled receptor (GPCR). The primary endogenous agonists of HCA₃ are 3-hydroxyoctanoic acid and kynurenic acid. HCA₃ is also a low-affinity biomolecular target for niacin (aka nicotinic acid).