The Mediterranean Sea is especially prone to the introduction of alien species due to an intense marine traffic, the connection with the Red Sea through the Suez Canal and intensive aquaculture. Catalonia, a region in the Northwestern Mediterranean, began an extensive study on the presence, distribution and impact of invasive macroalgae in 1992, which was extended to all macrobenthic alien species by 2007. Gathering all presence and abundance data of introduced species from the monitoring, we also calculated a Biopollution Level (BPL) index to assess the magnitude of the effects of introduced species on the marine biota at a local level (water body) as required by Marine Strategy Framework Directive (MSFD). Seventeen alien species have been identified although only three can be considered so far as threatening in non-modified environments: the green alga Caulerpa cylindracea and the red algae Womersleyella setacea and Asparagopsis armata. These species show an uneven distribution along the coast but sometimes coexist in the same water body. The impact of alien species on native communities was never severe as shown by the low values obtained using the BPL. The only species triggering a moderate to strong impact was Caulerpa cylindracea but it only affected a single water body. However, C. cylindracea exhibited a great temporal variability on its abundance, with sudden collapses of its populations, which also caused a great variability in the BPL. Future monitoring of the coasts of Catalonia is advised as there is an increase in the number of water bodies affected by alien species and an increase in their abundances from 2007 to 2012.