Simple SummaryThe shrimpPenaeus aztecus, native to the western Atlantic, was first reported in the Mediterranean Sea (Bay of Antalya, Southern Turkey) in 2010. In the following years, it proved its invasiveness with multiple records from all over the Mediterranean except the westernmost sector and the North Adriatic Sea. Several pieces of evidence suggest that the unintentional transport of larvae in the ballast waters of transoceanic vessels departing from the U.S. West Coast, instead of the escape of adults from unreported experimental shrimp farming, is the more likely pathway of the introduction ofP. aztecusin the Mediterranean Sea. The accurate scrutiny of scientific literature on non-indigenous species brought to light an earlier (2005) arrival in the Black Sea, which passed unnoticed as the shrimps were misidentified asPenaeus semisulcatus, also a non-indigenous species, which is established and exploited in the Levant Sea since 90 years. But it is native to the Indo-Pacific region, other misidentifications were also found, therefore morphological characters allowing correct identification of the two species and of the autochthonousPenaeus kerathurusare illustrated. Non-indigenous species are among the descriptors adopted in the Marine Strategy Framework Directive for determining the good environmental status of marine waters in the European States, hence the importance of their correct identification.AbstractThe shrimpPenaeus aztecus, native to the western Atlantic, was first reported in the eastern Mediterranean Sea in 2010. New records, from different Mediterranean localities, multiplied in the following years. The accurate search of the literature on non-indigenous species discovered it was misidentified more than once as another alien shrimp,P. semisulcatus, native to the Indo-Pacific region, with the result that its earlier presence in the Black Sea went unnoticed. Morphological characteristics allowing the identification of these two species, the autochthonousP. kerathurusand two other alienPenaeusspecies present in the Mediterranean, are reprised. The present distribution ofP. aztecusbased on literature records and surveys carried out in the northern and central Adriatic between 2016 and 2021 is mapped. The unintentional transport of larvae carried in ballast water by transoceanic vessels departing from the U.S. East Coast is suggested as the most probable introduction pathway. The significance of the correct identification of non-indigenous species, a “Descriptor” adopted in the Marine Strategy Framework Directive for determining the good environmental status of marine waters in the European States, is emphasized.