Understanding temporal trends in microplastic deposition in marine sedimentary environments can help to determine relationships between microplastics and sedimentary processes, as well as potential sources. Here we measured microplastic concentrations in three 210Pb dated sediment cores to characterize temporal changes in microplastic deposition in sediments along the Pacific Coast of Canada, within the Clayoquot Sound UNESCO Biosphere Reserve and the Pacific Rim National Park Reserve. Sediment grain size distribution and percent organic carbon content (%Corg) of the sediments were measured to better understand the sedimentary environment at the sites, and to examine potential relationships between sedimentary environment and microplastic deposition. While all microplastic types were considered, the microplastics discovered in this study were all microfibers, which corroborates other research in the area placing high quantities of microfibers in the subsurface seawater of the northeastern Pacific Ocean. In the surface 10 cm of sediment (where most microplastics were deposited), mean (+/- SE) microplastic concentrations (#particles/kg) in Clayoquot Sound were 288 ± 92 particles/kg, which is of a similar order of magnitude to 79% of sites studied in a metanalysis of 42 sites around the globe. Microplastic concentrations were not evenly distributed throughout the core and displayed a high degree of variability at depth. While few studies have measured microplastic accumulation rates in marine sedimentary environments, mean values measured over the last 30 years in Clayoquot Sound are on a similar order of magnitude to those documented for the same time period at all other sites that have reported microplastic accumulation rates globally, ranging from 2-35 particles/100cm2/year. Mean (+/- SE) microplastic accumulation rates in the region increased from 7 ± 3 particles/100cm2/year in 1950 to 33 ± 12 particles/100cm2/year in 2016, with increases coinciding with global plastic production and increases in aquaculture tenures in Clayoquot Sound. A significant relationship was observed between microplastic concentrations and accumulation rates and the %fine sediment grain size fraction, implying that microplastics may preferentially be deposited in finer sediments associated with lower energy environments.