Microplastics (MPs) defined as ‘small’ pieces of plastic < 5 mm have been found in almost every marine habitat around the world, and studies have shown that we can find them in the ocean surface, the water column, the seafloor, the shoreline, in biota and in the atmosphere-ocean interface. This study aimed to assess both marine and freshwater environments of Cocos Island, Costa Rica, in the Pacific Ocean, by sampling sediments and biota to determine the presence and abundance of this pollutant. Sediment samples were superficial and weighed one kilogram each. For the sampling of freshwater fish and shrimps, nonselective capture with small nets was made in rivers with access by land, while fishing rods were used for the marine fish sampling, and cage and scuba diving for lobsters. Plastics were found in all types of samples: 93% of marine sediments, 32% of freshwater sediments, 20% of freshwater fish, 15% of freshwater shrimps, 27% of marine fish, and 51% of marine lobsters. Like many reports around the world, it was expected to find MPs at marine samples, and it was concluded that ocean currents, tourism activities, and discarded fishing gear from illegal fishing activities could be the sources of marine pollutants. In contrast, the amount of MPs found in freshwater environments was not expected. Their possible sources are unclear at this moment.