Given the implications of microplastics contamination in aquatic ecosystems and information scarcity about microplastic abundances in estuarine sediments, this study aimed to quantify and describe the microplastics in the sublittoral sediments from Guanabara Bay. Sediment samples were collected at four sites and three months, microplastics were separated and classified according to type, color, size, and polymer composition. High abundances of microplastic (160 to 1000 items kg or 4367 to 25,794 items m) occurred independent of area or period, indicating microplastics are widely spread in Guanabara Bay. The dominant microplastic in the sediment was the translucent polyester microfiber of <1 mm size; which is a secondary microplastic, possibly coming from washing machines wastes. The extremely high availability of microplastics in Guanabara Bay, compared to the majority of studies around the world, suggests high risk of contamination to benthic organisms and demersal fish, as they may be ingesting microplastics.