As a prime mover in Alzheimer’s disease (AD), microglial activation requires membrane translocation, integration, and activation of the metamorphic protein chloride intracellular channel 1 (CLIC1), which is primarily cytoplasmic under physiological conditions. However, the formation and activation mechanisms of functional CLIC1 are unknown. Here, we found that the human antimicrobial peptide (AMP) LL-37 promoted CLIC1 membrane translocation and integration. It also activates CLIC1 to cause microglial hyperactivation, neuroinflammation, and excitotoxicity. In mouse and monkey models, LL-37 caused significant pathological phenotypes linked to AD, including elevated amyloid-β, increased neurofibrillary tangles, enhanced neuronal death and brain atrophy, enlargement of lateral ventricles, and impairment of synaptic plasticity and cognition, while Clic1 knockout and blockade of LL-37-CLIC1 interactions inhibited these phenotypes. Given AD’s association with infection and that overloading AMP may exacerbate AD, this study suggests that LL-37, which is up-regulated upon infection, may be a driving force behind AD by acting as an endogenous agonist of CLIC1.