The accurate assessment of regional earthquake hazards requires well-constrained geophysical models of upper-plate deformation (coseismic) during megathrust earthquakes. Land-level change estimates using quantitative methods applied to microfossils (e.g., diatoms) found in stratigraphic records from tidal wetlands offer the most precise means to reconstruct coseismic subsidence. However, existing quantitative estimates of coseismic subsidence along the Cascadia subduction zone are limited by a lack of observational data. We expand on our current understanding of earthquake-related, land-level change by applying the first diatom Bayesian transfer function to fossil diatom assemblages from a stratigraphic sequence of six, prehistoric earthquakes at Willapa Bay, Washington, USA. Reconstructed estimates range from 1.41 to 2.12 (± 0.69 to 0.83) m of cosesimic subsidence over the past 3500 years. Future work will continue to explore Cascadia’s past rupture behavior.