Adiposopathy is a pathological adipose tissue (AT) response to overfeeding characterized by reduced AT expandability due to impaired adipogenesis, which favors inflammation, insulin resistance (IR), and abnormal glucose regulation. However, it is unclear whether defective adipogenesis causes metabolic derangement also independently of an increased demand for fat storage. As galectin-3 has been implicated in both adipocyte differentiation and glucose homeostasis, we tested this hypothesis in galectin-3 knockout (Lgal3-/-) mice fed a standard chow. In vitro, Lgal3-/- adipocyte precursors showed impaired terminal differentiation (maturation). Two-month-old Lgal3-/- mice showed impaired AT maturation, with reduced adipocyte size and expression of adipogenic genes, but unchanged fat mass and no sign of adipocyte degeneration/death or ectopic fat accumulation. AT immaturity was associated with AT and whole-body inflammation and IR, glucose intolerance, and hyperglycemia. Five-month-old Lgal3-/- mice exhibited a more mature AT phenotype, with no difference in insulin sensitivity and expression of inflammatory cytokines versus WT animals, though abnormal glucose homeostasis persisted and was associated with reduced β-cell function. These data show that adipogenesis capacity per se affects AT function, insulin sensitivity, and glucose homeostasis independently of increased fat intake, accumulation and redistribution, thus uncovering a direct link between defective adipogenesis, IR and susceptibility to diabetes.