The sympathetic nervous system drives brown and beige adipocyte thermogenesis through the release of noradrenaline from local axons. However, the molecular basis of higher levels of sympathetic innervation of thermogenic fat, compared to white fat, has remained unknown. Here we show that thermogenic adipocytes express a previously unknown, mammal-specific protein of the endoplasmic reticuhim that we term calsyntenin 3 beta. Genetic loss or gain of expression of calsyntenin 3 beta in adipocytes reduces or enhances functional sympathetic innervation, respectively, in adipose tissue. Ablation of calsyntenin 3 beta predisposes mice on a high-fat diet to obesity. Mechanistically, calsyntenin 3 beta promotes endoplasmic-reticulum localization and secretion of S100b-a protein that lacks a signal peptide-from brown adipocytes. S100b stimulates neurite outgrowth from sympathetic neurons in vitro. A deficiency of S100b phenocopies deficiency of calsyntenin 3 beta, and forced expression of S100b in brown adipocytes rescues the defective sympathetic innervation that is caused by ablation of calsyntenin 3 beta. Our data reveal a mammal-specific mechanism of communication between thermogenic adipocytes and sympathetic neurons.