Technical Abstract: Synucleins are a family of homologous proteins principally known for their involvement in neurodegeneration. In neurons a-synuclein promotes assembly of SNARE complexes required for fusion of synaptic vesicles with the plasma membrane during neurotransmitter release. Y-synuclein is highly expressed in human white adipose tissue (WAT) and increased in obesity. Here we show that Y-synuclein is nutritionally regulated in WAT whilst its loss protects mice from high fat diet (HFD)-induced obesity and associated metabolic complications. Compared to HFD-fed wild type mice, HFD-fed Y-synuclein null mutant mice display increased lipolysis, lipid oxidation and energy expenditure, and reduced adipocyte hypertrophy. Knockdown of Y-synuclein in cultured adipocytes causes redistribution of the key lipolytic enzyme ATGL to lipid droplets and increases lipolysis. Y-synuclein deficient adipocytes also contain fewer SNARE complexes of a type involved in lipid droplet fusion. We show that Y-synuclein interacts directly with SNAP-23, a component of these SNARE complexes. However, treatment of cells with oleate disrupts this interaction, consistent with a role for Y-synuclein in delivering SNAP-23 to the SNARE complexes under this lipogenic condition. Thus, we propose that via these dual roles Y-synuclein may co-ordinately affect both lipolysis and lipid droplet formation depending on nutritional status. Our data reveal Y-synuclein as a novel regulator of lipid handling in adipocytes, whose function is particularly important in conditions of nutrient excess.