Cryo-electron microscopy structures and mass spectrometry analyses show that TAR DNA-binding protein 43 (TDP-43) forms amyloid filaments with a distinct fold in type A frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP) compared with TDP-43 filaments in type B FTLD-TDP and amyotrophic lateral sclerosis. Here, TARDBP is linked to frontotemporal dementia.