Glucose-dependent insulinotropic polypeptide (GIP) is best known as an incretin hormone that is secreted from K-cells of the proximal intestine, but evidence also implicates a role for GIP in regulating lipid metabolism and adiposity. It is well established that GIP receptor knockout (GIPR KO) mice are resistant to diet-induced obesity; however, the factors mediating this effect remain unresolved. Accordingly, we aimed to elucidate the mechanisms leading to adiposity resistance in GIPR KO mice with a focus on whole-body energy balance and lipid metabolism in adipose tissues. Studies were conducted in age-matched male GIPR KO and wild type (WT) mice fed a high-fat diet for 10 weeks. GIPR KO mice gained less body weight and fat mass compared to WT littermates, and this was associated with increased energy expenditure but no differences in food intake or faecal energy loss. Upon an oral lipid challenge, fatty acid storage in inguinal adipose tissue was significantly increased in GIPR KO compared with WT mice. This was not related to differential expression of lipoprotein lipase in adipose tissue. Adipose tissue lipolysis was increased in GIPR KO compared with WT mice, particularly following b-adrenergic stimulation, and could explain why GIPR KO mice gain less adipose tissue despite increased rates of fatty acid storage in inguinal adipose tissue. Taken together, these results suggest that the GIPR is required for normal maintenance of body weight and adipose tissue mass by regulating energy expenditure and lipolysis.