Relaxin-3, also known as insulin-like peptide-7, is a newly-identified peptide of the insulin superfamily. All members of this superfamily have a similar structure, which consists of two subunits (A-chain and B-chain) linked by disulfide bonds. Relaxin-3 is so named because it has a motif that can interact with the relaxin receptor. By contrast to other relaxins, relaxin-3 is mainly expressed in the brain and testis. In rodent brain, anatomical studies have revealed its predominant expression in neurons of the nucleus incertus of the dorsal pons, and a few other regions of the brainstem. On the other hand, relaxin-3-expressing nerve fibers and the relaxin-3 receptors, RXFP3 and RXFP1, are widely distributed in the forebrain, with the hypothalamus being one of the most densely-innervated regions. Therefore, relaxin-3 is considered to exert various actions through its ligand-receptor system. This minireview describes the expression of relaxin-3 in the brain, as well as its functions in the hypothalamus, including the stress response and food intake.