Objective— Obese adipose tissue shows hallmarks of chronic inflammation, which promotes the development of metabolic disorders. The mechanisms by which immune cells interact with each other or with metabolism-associated cell types, and the players involved, are still unclear. The CD40-CD40L costimulatory dyad plays a pivotal role in immune responses and in diseases such as atherosclerosis and may therefore be a mediator of obesity. Here we investigated whether CD40L is involved in adipose tissue inflammation and its associated metabolic changes. Methods and Results— To assess a putative role of CD40L in obesity in vivo, we evaluated metabolic and inflammatory consequences of 18 weeks of high-fat feeding in CD40L ^+/+ and CD40L ^−/− mice. In addition, C57Bl6 mice were injected with neutralizing anti-CD40L (αCD40L) antibody for 12 weeks while being fed a high-fat diet. Genetic deficiency of CD40L attenuated the development of diet-induced obesity, hepatic steatosis, and increased systemic insulin sensitivity. In adipose tissue, it impaired obesity-induced immune cell infiltration and the associated deterioration of glucose and lipid metabolism. Accordingly, αCD40L treatment improved systemic insulin sensitivity, glucose tolerance, and CD4 ⁺ T-cell infiltration in adipose tissue with limited effects on adipose tissue weight. Conclusion— CD40L plays a crucial role in the development of obesity-induced inflammation and metabolic complications.