Background: Tauopathies such as Alzheimer’s disease (AD) are characterized by abnormal hyperphosphorylation of the microtubule-associated protein tau (MAPT) aggregating into neurofibrillary tangles (NFTs). O-linked \beta-N-acetylglucosamine (O-GlcNAc) modifications have been suggested to regulate tau phosphorylation and aggregation and N-acetylglucosaminidase (OGA) removes GlcNAc moieties from proteins. Methods: We investigated effects of the OGA inhibitor Thiamet G in rTg4510 primary neuronal cultures and in rTg4510 mice. The rTg4510 mice overexpress human tau harboring the P301L mutation and display an age-dependent progression of tau pathology including hyperphosphorylated tau species and NFTs. Aged rTg4510 mice exhibit a non-mnemonic behavioral defect involving a hyperactive phenotype that is associated with the progression of tau pathology. Results: Thiamet G increased overall O-GlcNAc levels and crossed the blood brain barrier in rTg4510 mice. The free fraction of Thiamet G in the brain was 22-fold above the half maximal effective concentration (EC{}_{50}) measured in rTg4510 primary neurons. Chronic Thiamet G treatment (18 weeks) initiated in young 6 week old rTg4510 mice increased brain O-GlcNAc levels and this corresponded with a significant reduction in soluble and insoluble hyperphosphorylated tau in aged 24 week old rTg4510 mice. Levels of normally phosphorylated P301L tau were not altered under these conditions. Reduction of hyperphosphorylated tau species by increased O-GlcNAcylation was associated with significant attenuation of hyperactivity in 24 week old rTg4510 mice. Conclusions: Our findings support the pharmacological inhibition of OGA as a potential therapeutic approach for the treatment of AD and other tauopathies.