Alzheimer's disease (AD) is the most prevalent neurodegenerative disease in the elderly. Zinc (Zn) ion interacts with the pathogenic hallmark, amyloid-β (Aβ), and is enriched in senile plaques in brain of AD patients. To understand Zn-chelated Aβ (ZnAβ) species, here we systematically characterized ZnAβ aggregates by incubating equimolar Aβ with Zn. We found ZnAβ40 and ZnAβ42 both form spherical oligomers with a diameter of ~12-14 nm composed of reduced β-sheet content. Oligomer assembly examined by analytical ultracentrifugation, hydrophobic exposure by BisANS spectra, and immunoreactivity of ZnAβ and Aβ derived diffusible ligands (ADDLs) are distinct. The site-specific 13C labeled solid-state NMR spectra showed that ZnAβ40 adopts β-sheet structure as in Aβ40 fibrils. Interestingly, removal of Zn by EDTA rapidly shifted the equilibrium back to fibrillization pathway with a faster kinetics. Moreover, ZnAβ oligomers have stronger toxicity than ADDLs by cell viability and cytotoxicity assays. The ex vivo study showed that ZnAβ oligomers potently inhibited hippocampal LTP in the wild-type C57BL/6JNarl mice. Finally, we demonstrated that ZnAβ oligomers stimulate hippocampal microglia activation in an acute Aβ-injected model. Overall, our study demonstrates that ZnAβ rapidly form toxic and distinct off-pathway oligomers. The finding provides a potential target for AD therapeutic development.