Significance The tau protein is involved in Alzheimer’s and other neurodegenerative diseases, where the location, morphology, and quantity of amyloid fibrils composed of tau correlate with the disease type and stage. While tau fibrillary aggregates have been colocalized in brains together with several cofactors, their role in fibril formation, structure, and seeding has been largely neglected. We show that seeding of tau aggregation is facilitated by polyanionic cofactors, and that seeded or recombinant mature fibrils depolymerize into monomers when their cofactor is removed. We show that cofactor-assisted seeding with mouse brain-derived tau fibrils yielded tau fibrils with distinct and narrowed structural properties compared with heparin-induced fibrils, suggesting that the fibrillar templates tuned the structure of the seeded fibril.