Leptin is an adipocyte-derived hormone that plays a key role in energy homeostasis, yet resistance to leptin is a feature of most cases of obesity in humans and rodents. In vitro analysis suggested that the suppressor of cytokine signaling-3 (Socs3) is a negative-feedback regulator of leptin signaling involved in leptin resistance. To determine the functional significance of Socs3 in vivo, we generated neural cell–specific SOCS3 conditional knockout mice using the Cre–loxP system. Compared to their wild-type littermates, Socs3-deficient mice showed enhanced leptin-induced hypothalamic Stat3 tyrosine phosphorylation as well as pro-opiomelanocortin (POMC) induction, and this resulted in a greater body weight loss and suppression of food intake. Moreover, the Socs3-deficient mice were resistant to high fat diet–induced weight gain and hyperleptinemia, and insulin-sensitivity was retained. These data indicate that Socs3 is a key regulator of diet-induced leptin as well as insulin resistance. Our study demonstrates the negative regulatory role of Socs3 in leptin signaling in vivo, and thus suppression of Socs3 in the brain is a potential therapy for leptin-resistance in obesity.