Abstract: The lack of blood-brain barrier (BBB) penetrating ability has hindered the delivery of many therapeutic agents for tauopathy therapeutic treatment. A circular bifunctional aptamer reported here has been able to enhance the in vivo BBB penetration for improved therapy. The circular aptamer includes one transferrin receptor (TfR) aptamer to facilitate TfR-aptamer recognition-induced transcytosis across BBB endothelial cells, and one Tau protein aptamer selected to inhibit Tau phosphorylation and other tauopathy-related pathological events in the brain. This bispecific construct exhibits strong specificity towards Tau and enhanced plasma stability in comparison to linear Tau aptamer. In vivo administration of circular Tau-TfR aptamer results in a rapid uptake into relevant brain regions after crossing the BBB, such as hippocampus and cortex.