Neurofibrillary tangles (NFTs) in Alzheimer disease and related tauopathies are composed of insoluble hyperphosphorylated Tau protein, but the mechanisms underlying the conversion of highly soluble Tau into insoluble NFTs remain elusive. Here, we demonstrate that introduction of minute quantities of misfolded preformed Tau fibrils (Tau pffs) into Tau-expressing cells rapidly recruit large amounts of soluble Tau into filamentous inclusions resembling NFTs with unprecedented efficiency, suggesting a "seeding"-recruitment process as a highly plausible mechanism underlying NFT formation in vivo. Consistent with the emerging concept of prion-like transmissibility of disease-causing amyloidogenic proteins, we found that spontaneous uptake of Tau pffs into cells is likely mediated by endocytosis, suggesting a potential mechanism for the propagation of Tau lesions in tauopathy brains. Furthermore, sequestration of soluble Tau by pff-induced Tau aggregates attenuates microtubule overstabilization in Tau-expressing cells, supporting the hypothesis of a Tau loss-of-function toxicity in cells harboring NFTs. In summary, our study establishes a cellular system that robustly develops authentic NFT-like Tau aggregates, which provides mechanistic insights into NFT pathogenesis and a potential tool for identifying Tau-based therapeutics.