Gestational Diabetes Mellitus (GDM) increases the long-term risk for metabolic and cardiovascular diseases in the offspring. However, the underlying mechanisms are not well understood. This study investigates the impact of GDM on fetoplacental vascular function and molecular mechanisms underlying endothelial dysfunction. Clinical data and tissue samples were collected from normoglycemic (NG, n = 33) and GDM (n = 19) pregnancies. Offspring in the GDM group were delivered earlier, had a larger placental size, and had a reduced placental efficiency. Functional analysis using a Mulvany myograph demonstrated a significant impairment of insulin-mediated vasodilation in fetoplacental vessels of GDM patients compared to NG controls. This vascular dysfunction was associated with a reduction in total insulin receptor protein expression. Further investigation revealed an impaired PI3K/AKT/eNOS signaling pathway, as endothelial cells from GDM pregnancies showed a deficient insulin-induced phosphorylation of AKT. These results indicate that maternal GDM induces insulin resistance and endothelial dysfunction in the fetoplacental vasculature through impairment of the AKT/eNOS pathway, providing a key mechanism for its adverse neonatal outcomes and the increased lifelong cardiovascular risk in the offspring.