The inappropriate disposal of plastics causes negative impacts on the environment. Despite being resistant, plastic is subject to degradation and fragmentation processes which result in the formation of microplastics (MPs, i.e. particles ¡ 5 mm). Currently, MPs are considered emerging contaminants and are virtually present in all aquatic ecosystems. The Mirim Lagoon is the second largest lagoon in South America, located on the border with Uruguay and its hydrographic basin is economically important for activities focused mainly on agriculture, fishing, irrigation and human supply. Despite of these anthropogenic activities, there are no research papers on plastic contamination in this environment. In order to characterize the potential sediment contamination with MPs of Mirim Lagoon, a gravity corer was used to sampling the corer in December 2018. In the laboratory, the core was sagittally opened. The sediment for MPs evaluation was carried out every 2 cm along the core. Then, sediments were dried in an oven at 40°C until constant weight. The isolation of potential MPs from the sediment was carried out using a saline flotation method using a NaCl solution (density 1.2 g.cm-³), followed by filtration of the supernatant using a filter paper (0.7 µm pore). The filters were analyzed under a stereomicroscope attached to a camera. Potential MPs were visually identified and classified according to type (fiber, fragment, pellet) and color. A total of 41 potential MPs were recorded in the top 80 cm of sedimentary sequence, being mostly fibers and blue in color. MPs distribution showed no contamination in the basal section of the core. On the 20 cm layer, a distinct increase in contamination was detected. Further work will involve determination of the polymer types and sediment age in order to stablish an Anthropocene marker for a better understanding of MP contamination in South America.