Glycogen synthase kinase-3 (GSK-3) is linked to the pathogenesis of Alzheimer's disease (AD), senile plaques (SPs), and neurofibrillary tangles (NFTs), but the specific contributions of each of the GSK-3 α and β isoforms to mechanisms of AD have not been clarified. In this study, we sought to elucidate the role of each GSK-3α and GSK-3β using novel viral and genetic approaches. First, we developed recombinant adeno-associated virus 2/1 short hairpin RNA constructs which specifically reduced expression and activity of GSK-3α or GSK-3β. These constructs were injected intraventricularly in newborn AD transgenic (tg) mouse models of SPs (PDAPP⁺/⁻), both SPs and NFTs (PDAPP⁺/⁻;PS19⁺/⁻), or wild-type controls. We found that knockdown (KD) of GSK-3α, but not GSK-3β, reduced SP formation in PDAPP⁺/⁻ and PS19⁺/⁻;PDAPP⁺/⁻ tg mice. Moreover, both GSK-3α and GSK-3β KD reduced tau phosphorylation and tau misfolding in PS19⁺/⁻;PDAPP⁺/⁻ mice. Next, we generated triple tg mice using the CaMKIIα-Cre (α-calcium/calmodulin-dependent protein kinase II-Cre) system to KD GSK-3α in PDAPP⁺/⁻ mice for further study of the effects of GSK-3α reduction on SP formation. GSK-3α KD showed a significant effect on reducing SPs and ameliorating memory deficits in PDAPP⁺/⁻ mice. Together, the data from both approaches suggest that GSK-3α contributes to both SP and NFT pathogenesis while GSK-3β only modulates NFT formation, suggesting common but also different targets for both isoforms. These findings highlight the potential importance of GSK-3α as a possible therapeutic target for ameliorating behavioral impairments linked to AD SPs and NFTs.