Tau aggregates are critical pathological features of Alzheimer’s disease (AD) and other tauopathies. Growing evidence suggests that soluble tau aggregates trigger neurodegenerative phenotypes. However, the nature of the tau species and interactors involved in its aggregation and spreading remains unclear. By using size exclusion chromatography, mass spectrometry, and bioinformatic analysis, we identified Bassoon protein (BSN) as a significant tau interactor in PS19 mice, as well as in human AD and PSP cases. We also found that overexpression of BSN triggers the aggregation of tau and increase the tau seeding activity in vitro, and also exacerbates the degenerative phenotype in a Fly model for tauopathy. Knockdown of BSN significantly reduced tau spreading in PS19 mouse brains and destabilized the tau aggregates, leading to a reduction in the tau pathology in this model. Furthermore, BSN downregulation was able to restore the neurodegenerative phenotype in PS19 mice, observed in electrophysiology and behavioral tests. Our results identify BSN as a key interactor of tau spread and aggregation in the brain, and therefore a potential target for the treatment of diseases that involve tau spread and aggregation.