Microplastics (polymer particles less than 5 mm in size) are ubiquitous contaminants in aquatic environments, and there is growing concern about their effects on environmental and human health. Despite the risks of this class of contaminant, works in this area in Rio de Janeiro are still incipient and a correct assessment of the presence and distribution of microplastics is necessary. Based on this scenario, the main aims of the present work are: (i) to evaluate the occurrence and dispersion of microplastics between Guanabara Bay and Natural Monument (MoNa) of the Cagarras Islands; and (ii) quantitatively and qualitatively determining the collected microplastic. The material counted and separated into classes was taken for further identification with FTIR-ATR and surface characterization via SEM-EDS. The results show a higher density of microplastic in the points closest to Guanabara Bay, with a density gradient being observed along the points. In two campaigns there was a greater abundance of items smaller than 1 mm (51.0 ± 5.9 percent), while in two other campaigns, the greatest abundance corresponded to the size class 1