Outer membrane vesicles (OMVs) are nanosized vesicles produced by the gut microbiota (GM). The GM is well‐known to be involved in the pathological process of Alzheimer's disease (AD). However, the mechanism of OMVs is not clear. In the present study, we demonstrated the involvement of OMVs in the development of cognitive (learning and memory) dysfunction induced by blood–brain barrier (BBB) disruption. More important, further study showed that OMVs induced tau phosphorylation by activating glycogen synthase kinase 3β (GSK‐3β) in the hippocampus. OMVs activated astrocytes and microglia, increased secretion of inflammatory cytokines (nuclear factor κB, interleukin‐1β, and tumour necrosis factor‐α) in the hippocampus. Therefore, OMVs increase the permeability of the BBB and promote the activation of astrocytes and microglia, inducing an inflammatory response and tau hyperphosphorylation by activating the GSK‐3β pathway and finally leading to cognitive impairment. In this study, we demonstrate that outer membrane vesicles (OMVs) derived from the faeces of Alzheimer's disease (AD) patients enter the brain, break the blood–brain barrier (BBB), induce marked neuroinflammation and specifically aggravate tau hyperphosphorylation, thus potentially inducing behavioural cognitive impairment