Alzheimer's disease is characterized by the deposition of amyloid-beta as extracellular plaques and hyperphosphorylated tau as intracellular neurofibrillary tangles. Tau pathology characterizes not only Alzheimer's disease, but also many other tauopathies, presenting tau as an attractive therapeutic target. Passive tau immunotherapy has been previously explored; however, because only a small fraction of peripherally delivered antibodies crosses the blood-brain barrier, enters the brain and engages with tau that forms intracellular aggregates, more efficient ways of antibody delivery and neuronal uptake are warranted. In the brain, tau exists as multiple isoforms. Here, we investigated the efficacy of a novel 2N tau isoform-specific single chain antibody fragment, RN2N, delivered by passive immunization in the P301L human tau transgenic pR5 mouse model. We demonstrate that, in treated mice, RN2N reduces anxiety-like behaviour and phosphorylation of tau at distinct sites. When administration of RN2N was combined with focused ultrasound in a scanning mode (scanning ultrasound), RN2N delivery into the brain and uptake by neurons were markedly increased, and efficacy was significantly enhanced. Our study provides evidence that scanning ultrasound is a viable tool to enhance the delivery of biologics across the blood-brain barrier and improve therapeutic outcomes and further presents single-chain antibodies as an alternative to full-length antibodies.