Maternal high-fat diet (MHFD) during pregnancy induces social and synaptic deficits in offspring, which are linked to alterations in the gut microbiome. The study shows that MHFD offspring exhibit impaired social behavior and gut microbiota dysbiosis, which can be reversed by co-housing with offspring of mothers on a regular diet (MRD) or by transferring microbiota from MRD mice to germ-free (GF) mice. A single commensal bacterial species, Lactobacillus reuteri, restores oxytocin levels, synaptic function, and social behavior in MHFD offspring. These findings suggest that maternal diet affects offspring behavior through gut microbial imbalance, and that probiotic treatment may alleviate behavioral abnormalities associated with neurodevelopmental disorders. The study highlights the role of the gut-brain axis in social behavior and demonstrates that microbial reconstitution can reverse MHFD-induced deficits. The results indicate that L. reuteri improves social behavior by enhancing oxytocin levels and synaptic plasticity in the ventral tegmental area (VTA), which is crucial for social interactions. The findings provide a causal link between maternal diet, gut microbiota, and behavioral outcomes, suggesting that targeted probiotic interventions could be a potential non-invasive treatment for neurodevelopmental disorders.Maternal high-fat diet (MHFD) during pregnancy induces social and synaptic deficits in offspring, which are linked to alterations in the gut microbiome. The study shows that MHFD offspring exhibit impaired social behavior and gut microbiota dysbiosis, which can be reversed by co-housing with offspring of mothers on a regular diet (MRD) or by transferring microbiota from MRD mice to germ-free (GF) mice. A single commensal bacterial species, Lactobacillus reuteri, restores oxytocin levels, synaptic function, and social behavior in MHFD offspring. These findings suggest that maternal diet affects offspring behavior through gut microbial imbalance, and that probiotic treatment may alleviate behavioral abnormalities associated with neurodevelopmental disorders. The study highlights the role of the gut-brain axis in social behavior and demonstrates that microbial reconstitution can reverse MHFD-induced deficits. The results indicate that L. reuteri improves social behavior by enhancing oxytocin levels and synaptic plasticity in the ventral tegmental area (VTA), which is crucial for social interactions. The findings provide a causal link between maternal diet, gut microbiota, and behavioral outcomes, suggesting that targeted probiotic interventions could be a potential non-invasive treatment for neurodevelopmental disorders.