Microplastics are an ecological stressor with implications for ecosystem and human health when present in seafood. We quantified microplastic types, concentrations, anatomical burdens, geographic distribution, and temporal differences in Pacific oysters (Crassostrea gigas) and Pacific razor clams (Siliqua patula) from 15 Oregon coast, U.S.A. sites. Organisms were chemically digested and visually analyzed for microplastics, and material type was determined using Fourier Transform Infrared Spectroscopy. Microplastics were present in organisms from all sites. On average, whole oysters and razor clams contained 10.95 ± 0.77 and 8.84 ± 0.45 microplastic pieces per individual, or 0.35 ± 0.04 and 0.16 ± 0.02 pieces g-1 tissue, respectively. Contamination was quantified but not subtracted. Over 99% of microplastics were fibers. Spring samples contained more microplastics than summer in oysters but not razor clams. This study provides a spatially extensive baseline of microplastics in Oregon bivalves and is the first to determine Pacific razor clam concentrations.