High-fat diet (HFD) and obesity are associated with cognitive dysfunction, depression, and increased anxiety, impacting emotional and social behavior. Additionally, HFD affects the immune system, leading to low-level chronic inflammation involving microglia, the brain’s resident immune cells. Inhibiting microglia alters social behavior, and improves HFD-induced metabolic effects. Hence, this raises the question of whether microglial cells mediate HFD-dependent changes in behavioral outcomes.To investigate the effect of HFD-driven microglial activation on neuronal function, HFD adult CX3CR1GFP/+ male mice were fed a HFD or a control diet (CC) for six weeks. Mice were first subjected to a 3-chamber test, to assess their social behavior, and this revealed that HFD mice displayed alterations compared to CC mice. Then, to understand if microglia contributed to the observed behavioral modulation, we examined neuronal c-Fos activation, and microglial cell density and morphology, in brain areas differentially activated by the social task.Additionally, Top of Form to explore the molecular basis of the observed microglial changes, we conducted an RNA-seq experiment on microglial cells isolated from a second cohort of mice, revealing significant remodeling of the microglia-specific transcriptome following HFD feeding.Cellular and functional investigations of in vivo microglial dynamics through 2-photon imaging demonstrated a HFD-specific effect on microglial cell motility.Overall, this study shows that HFD dramatically influences microglial cells both transcriptionally and functionally, with potential consequences on mouse behavior.