The accumulation of amyloid-β (Aβ) and overactivation of microglia contribute to the pathogenesis of Alzheimer's disease (AD), but the interaction between microglial activation and Aβ deposition in AD remains elusive. Here we revealed that Aβ activates microglia and promotes the release of Galectin-9 (Gal-9), a member of the β-galactoside-binding family of lectins. The levels of Gal-9 in the cerebrospinal fluid and brain tissues of AD patients are higher than those in control subjects. Gal-9 interacts with Aβ and promotes its aggregation, generating Gal-9-Aβ fibrils with enhanced seeding activity and neurotoxicity. The expression of Gal-9 increases with age in the brains of APP/PS1 transgenic mice. Knockout of Gal-9 in APP/PS1 mice substantially reduced Aβ sedimentation, neuroinflammation, and cognitive impairment. Moreover, depletion of Gal-9 inhibited the seeding activity of brain homogenates from APP/PS1 mice. These findings reveal a mechanism by which microglia-derived Gal-9 accelerates Aβ aggregation and seeding in AD. Thus, strategies aimed at inhibiting Gal-9 may hold promise as a disease-modifying therapy to alleviate AD pathology.