The widespread use of plastic products in our modern life represents a serious threat to aquatic environments and wild animals that are exposed to plastic waste. Although microplastics (MPs) have been reported in fish from several freshwater environments around the world, mountain environments have been little studied so far. The occurrence of MPs was assessed in the gastrointestinal tracts (GITs) of non-native (rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta) and native (torrent catfish Hatcheria macraei) fish from the Mendoza River in the Central Andes, Argentina. Fibers (85%) were the main MPs type recovered from the fish here analyzed, followed by fragments (15%). Blue fibers were the main type of MPs in analyzed specimens: brown trout (50%), rainbow trout (71%), and torrent catfish (63%). Significant differences in the median total MPs’ abundance and median total fiber abundance were observed among fish species. The highest MPs’ abundance was found in the GITs of brown trout followed by rainbow trout, while the lowest was found in the GITs of torrent catfish. This study represents a baseline for the occurrence and characteristics in terms of shape and color of MPs in freshwater fish collected from a mountain river of the Central Andes.