Author Summary Genome-wide linkage analyses have revealed that an STS marker D6S1009 (about 55 kb from the SLC35D3 gene) is linked to obesity or BMI in the Framingham Heart Study, but its genetic entity is unknown. Here we characterized the features of obesity and metabolic syndrome with reduced physical activity levels in a previously identified ros mouse mutant, carrying a homozygous Slc35d3 mutation. These ros phenotypes were caused by the intracellular accumulation of D1R mostly on ER in the striatal neurons, impairing D1R signaling and reducing energy expenditure. In addition, we identified two mutations of SLC35D3 in patients with metabolic syndrome which are subcellularly mislocalized. We propose that the SLC35D3 gene is likely a novel candidate gene for MetS and obesity. Obesity is one of the largest health problems facing the world today. Although twin and family studies suggest about two-thirds of obesity is caused by genetic factors, only a small fraction of this variance has been unraveled. There are still large numbers of genes to be identified that cause variations in body fatness and the associated diseases encompassed in the metabolic syndrome (MetS). A locus near a sequence tagged site (STS) marker D6S1009 has been linked to obesity or body mass index (BMI). However, its genetic entity is unknown. D6S1009 is located in the intergenic region between SLC35D3 and NHEG1. Here we report that the ros mutant mice harboring a recessive mutation in the Slc35d3 gene show obesity and MetS and reduced membrane dopamine receptor D1 (D1R) with impaired dopamine signaling in striatal neurons. SLC35D3 is localized to both endoplasmic reticulum (ER) and early endosomes and interacts with D1R. In ros striatal D1 neurons, lack of SLC35D3 causes the accumulation of D1R on the ER to impair its ER exit. The MetS phenotype is reversible by the administration of D1R agonist to the ros mutant. In addition, we identified two mutations in the SLC35D3 gene in patients with MetS, which alter the subcellular localization of SLC35D3. Our results suggest that the SLC35D3 gene, close to the D6S1009 locus, is a candidate gene for MetS, which is involved in metabolic control in the central nervous system by regulating dopamine signaling.