Filamentous inclusions composed of the microtubule‐associated protein tau are a defining characteristic of a large number of neurodegenerative diseases. Here we show that tau degradation in stably transfected and non‐transfected SH‐SY5Y cells is blocked by the irreversible proteasome inhibitor lactacystin. Further, we find that in vitro, natively unfolded tau can be directly processed by the 20S proteasome without a requirement for ubiquitylation, and that a highly reproducible pattern of degradation intermediates is readily detectable during this process. Analysis of these intermediates shows that 20S proteasomal processing of tau is bi‐directional, proceeding from both N‐ and C‐termini, and that populations of relatively stable intermediates arise probably because of less efficient digestion of the C‐terminal repeat region. Our results are consistent with an in vivo role for the proteasome in tau degradation and support the existence of ubiquitin‐independent pathways for the proteasomal degradation of unfolded proteins.