Abstract: This dataset includes sediment porosity measurements from the continental margin offshore Washington State, USA. Samples were collected by the RV Atlantis on AT26-04 (Cruise DOI: 10.7284/900417). Push core and multi-core samples were collected adjacent to heat flow and pore water chemistry measurement locations along a transect from the upper slope to the incoming plate, including seep sites. The study was designed to evaluate the heat flow, fluid flow, and chemical flux distribution across the convergent margin.