This data release includes grain-size measurements of sediment samples collected from the substrate surface and uppermost 10 cm of sediment deposits in the Klamath estuary, northern California. Samples were collected using a BMH-60 bed-material sampler deployed from a boat, or by hand trowel from subaerial or shallow-water (less than 0.5 m water depth) regions along the estuary margins and side channels. Sediment grain size was analyzed at the U.S. Geological Survey (USGS) laboratory in Santa Cruz, Calif. Particles coarser than 2 mm were sieved using a RO-TAP sieve shaker, and particles finer than 2 mm were analyzed with a Coulter laser particle-size analyzer (LPSA), after organic matter had been removed using a hydrogen peroxide solution. At some submerged locations where sampling was attempted, no sample was recovered. The field crew observed this occurring in places where the substrate was too coarse-grained to be collected by the bed-material sampler (cobbles), where large particles were embedded in finer (poorly sorted) matrix, and where sediment was covered by algal mats thick enough to prevent the sampler from accessing the sedimentary substrate. Locations where sampling was attempted but no sample was recovered are indicated in the ‘Comments’ column within the data spreadsheet. The sediment samples were collected under a Cultural Resources Management Permit issued by the Yurok Tribe, whose support the USGS acknowledges gratefully.