During the present study an inventory was made of the alien benthic macroinvertebrates occurring in Flanders. To this end collections of biological samples were investigated and supplemented with own sampling campaigns. Three new macroinvertebrate species for Flanders were discovered. In total 72 alien macroinvertebrates were encountered of which 51 in fresh and brackish waters and 21 in the marine environment. Most alien macroinvertebrates belonged to the crustaceans and molluscs. Alien species found in the fresh and brackish water environment mainly originated from North-America and the Ponto-Caspian area (around the Black and Caspian Sea). This study shows that alien macroinvertebrates are widespread and abundantly present in many water types in Flanders. Harbours, canals and brackish polder watercourses can be considered as hotspots for alien macroinvertebrates. Based on observations in neighbouring countries, several additional species are expected to arrive in the near future. A follow-up of the established alien invasive species together with a monitoring scheme to detect new incoming high-risk species are necessary to estimate the size of the problem, to detect and control pathways and to be able to closely follow the ecological and economic impact of these species.