This paper confirms the occurrence of the blue crab, Callinectes sapidus, in a Portuguese estuary (Sado estuary), demonstrat-ing a full south-western extension of the species distribution along the Atlantic and Mediterranean coasts bordering the European continent. The present record associated with current and previous anecdotal occurrence reports suggests that the blue crab has established a population in the Sado estuary but with low census size. New geographical records of non-native species presence provide important information about spread rates, routes and vectors (Lockwood et al., 2007). The records of non-native species in terrestrial environments are more common and abundant than in marine environments (Ruiz et al., 2000). Among marine environments, coastal and estuarine regions are the most invaded, with decapod crustaceans being commonly reported as non-native invertebrates (Ruiz et al., 2000). One such species is the blue crab, Callinectes sapidus Rathbun, 1896. Blue crabs are native to the western Atlantic Ocean, ranging from Nova Scotia to Uruguay and inhabiting coastal waters and estuaries from the intertidal area to 90 m deep (Hines et al., 1987). The species is a known invader in the eastern Atlantic margin where its pres-ence dates back to the early 1900s with the first known record from Rochefort harbour, in north-western France (Bouvier, 1901). Later records in the mid-1900s came from several loca-tions in the North Sea and western Europe off Denmark, The Netherlands and Belgium (Wolff, 1954, and references therein; Maes et al., 1998). Records from off northern Spain (Gijón), in the Cantabrian region (Cabal et al., 2006), from western Portugal, in the Tagus estuary (Gaudêncio & Guerra, 1979), and from southern Iberia (Guadalquivir, WWF/ADENA, 2002 in Cabal et al., 2006) confirm the south-wards expansion of blue crabs along the estuaries of western Europe. The blue crab has also reached the Mediterranean Sea, with the earliest confirmed records dating to the 1940s (Nehring, 2011). Since then, the species dispersed throughout the eastern Mediterranean basin and Black Sea in the following decades (Galil, 2000, and references therein), and has recently reached the western basin (Castejón & Guerao, 2013). Interestingly, after the original record of C. sapidus off Portugal in the mid-1970s, no further blue crabs were detected in the Tagus estuary despite the intensive research surveys conducted (P. Chainho, personal communication). Thus, blue crabs were until recently considered as a non-established species in Portuguese territory (ICES, 2011). The present paper provides a new record on the occurrence of C. sapidus in another Portuguese estuary (Sado estuary), extending its distribution in south-western Europe and confirming the expansion of this species along western European estuaries.