TDP-43 proteinopathies are clinically and genetically heterogeneous diseases that had been considered distinct from classical amyloid diseases. Here, we provide evidence for the structural similarity between TDP-43 peptides and other amyloid proteins. AFM and EM examination of peptides spanning a previously defined amyloidogenic fragment revealed a minimal core region that forms amyloid fibrils similar to the TDP-43 fibrils detected in FTLD-TDP brain tissues. An ALS-mutant A315E amyloidogenic TDP-43 peptide is capable of cross-seeding other TDP-43 peptides and amyloid-β peptide. Sequential Nuclear Overhauser Effects (NOEs) and Double-quantum-filtered correlation spectroscopy (DQF-COSY) in NMR analyses of the A315E-mutant TDP-43 peptide indicate that it adopts an anti-parallel β conformation. Neuronal cultures in compartmentalized microfluidic-chambers demonstrate that the TDP-43 peptides can be taken up by axons and induce TDP-43 redistribution, axonotoxicity and neuronal death, thus, recapitulating key neuropathological features of TDP-43 proteinopathies. Importantly, one single amino acid change in the amyloidogenic TDP-43 peptide that disrupts fibril formation also eliminates neurotoxicity, supporting that amyloidogenesis is critical for TDP-43 neurotoxicity.