This work updates the characterization of winter fish communities in salt marsh areas of Guadiana estuary (SE-Portugal) and discusses the potential risks of habitat dominance by a non-indigenous species (NIS). To this effect, six field campaigns were carried out during winter season targeting the collection of fish species. Individuals from seven different families (marine and estuarine) were collected although the community was dominated by two estuarine species – the native Pomatoschistus sp. (goby) and the NIS Fundulus heteroclitus (mummichog). Goby controlled the majority of salt marsh habitats, except one specific, marsh pool, where extreme environmental conditions were registered, namely high temperature and salinity. Such conditions may have boosted the intrusion of mummichog in this area. This species is well adapted to a wide range of abiotic factors enabling them to colonize habitats where no predators inhabit. Impacts of mummichog intrusion in the Guadiana salt marsh area are still unpredictable since this is the first recorded in such high density. Nevertheless, in scenarios of increased anthropogenic pressure and, consequently, habitat degradation, there is a potential risk of mummichog spread to other habitats and therefore compete for space and food resources with native species.