Diabetes mellitus (DM) is a metabolic disease that can affect the central nervous system and behavioral traits in animals. Streptozotocin-induced diabetes is considered an autoimmune disease. The aim of the current study was to determine whether supplementation with the alcoholic extract of Avicennia marina leaves could improve diabetes-associated pathological changes. The animals were divided into four groups: a control group (A), an A. marina receiving nondiabetic group (B), a diabetic group (C), and a DM group orally supplemented with A. marina alcoholic leaf extract (D). The DM group of animals receiving the alcoholic extract of A. marina leaves had reduced blood glucose levels, improved blood picture, and organ functions. This group also showed improvement in locomotory behavior. The results of this study showed that supplementation with the alcoholic extract of A. marina leaves reduced oxidative stress and blood sugar levels, protected the liver, and improved the neurobehavioral changes associated with diabetes in mice. Introducing alcoholic leaf extract of A. marina to diabetic mice decreased inflammatory cells aggregation, vacuolation, and hemorrhage. Additionally, a positive effect of the alcoholic leaf extract on the histopathological changes was observed in the testicular tissue of treated mice.