In the European Union, regulations and policies have been established to prevent and manage invasive alien species (IAS). The EU Regulation 1143/2014/EC, commonly known as the IAS Regulation, identifies species that pose significant ecological risks, requiring a coordinated response across all EU Member States. These species are classified as “species of Union concern” based on thorough risk assessments and standardized procedures within the EU. Accordingly, species are included in The List of Invasive Alien Species of Union Concern (the Union list), which is subject to regular updates through continuous species evaluations. Currently, the majority of the species on this list are terrestrial and freshwater with few marine species represented. The primary aim of this paper is to review the procedure for the inclusion of species in the Union list. The steps for inclusion are outlined by the IAS Regulation and associated legislation, which govern the risk assessment process. In addition, this paper synthesizes available scientific literature and official documents to summarize the impacts of Plotosus lineatus and Rugulopteryx okamurae on biodiversity and ecosystem services. The potential for future updates to the Union list is currently under consideration, and some marine species are within.