Tauopathies are neurodegenerative diseases characterized by hyperphosphorylated tau (p-tau) and neurofibrillary tangles. Autophagy is a critical self-degradative mechanism for preserving cellular homeostasis and function, including the clearance of misfolded proteins. In tauopathies, autophagy is impaired, resulting in excessive accumulation of p-tau. Omipalisib, a selective mammalian target of rapamycin (mTOR) inhibitor, has been explored in phase I clinical trial with solid tumors or lymphoma. In this study, we aimed to effect of omipalisib on tauopathy in in vitro and in vivo. We observed that omipalisib increased protein expression of LC3B and decreased protein expression of p62 via inhibiting mTOR in a time-dependent manner in human tau (P301L) expressing SH-SY5Y stable cells. In addition, omipalisib reduced the expression of p-tau without cytotoxicity. Next, 6-month-old PS19 mice were administered with omipalisib for 2 months. We found that the levels of insoluble p-tau were decreased and spatial memory dysfunction was alleviated in omipalisib-treated PS19 mice. Overall, these results showed that omipalisib decreased the expression of p-tau via activation of mTOR-mediated autophagy, resulting in the amelioration of spatial memory deficit. This study suggests the potential of omipalisib as a candidate for tauopathies.