Microplastics (MP) known to be ubiquitous in the plastisphere, have yet to be quantified within Great Bay Estuary (GBE) in the Gulf of Maine region of North America. We extracted and analyzed MP in archived sediment cores obtained from representative transects across GBE. Results indicated that MP are distributed in GBE sediments, 0-30 cm, at an average occurrence of 100±50 particles g-1 and that morphology varies by site and depth. Analysis by sediment depth and age class indicated that MP deposition increased over several decades but recently (5-10 years) has likely begun to decrease. Hydrodynamic and particle transport modeling indicated that bed characteristics are a more controlling factor in MP distribution than MP properties and that the highest deposition likely occurs in regions with weaker hydrodynamic flows and lower bed shear stress e.g., eelgrass meadows and along the fringes of the Bay. These results provide a baseline and predictive understanding of the occurrence, morphology, and sedimentation of MP in the estuary.