Obesity is one of the major health problems of our times. Elucidating the signaling mechanisms by which high-fat caloric diet induces obesity is critical for the understanding of this condition and for the development of therapeutic strategies for its treatment. Here, we demonstrate a novel role for protein CD38 as a regulator of body weight during a high-fat diet. CD38 is a ubiquitous enzyme that catalyzes the synthesis of second messengers and has been implicated in the regulation of a wide variety of signaling pathways. We report that CD38-deficient mice are protected against high-fat diet-induced obesity owing to enhanced energy expenditure. In fact, calorimetric studies indicate that CD38-deficient animals have a higher metabolic rate compared to control mice. Analysis of the mechanism revealed that this resistance to diet-induced obesity is mediated at least in part via a NAD-dependent activation of SIRT-PGC1 axis, a well-established cascade, involved in the regulation of mitochondrial biogenesis and energy homeostasis. Thus, together these results identify a novel pathway regulating body weight and clearly show that CD38 is a nearly obligatory component of the cellular cascade that led to diet-induced obesity.--Barbosa, M. T. P., Soares, S. M., Novak, C. M., Sinclair, D., Levine, J. A., Aksoy, P., Chini, E. N. The enzyme CD38 (a NAD glycohydrolase, EC 3.2.2.5) is necessary for the development of diet-induced obesity.