The ctenophore Mnemiopsis leidyi A. Agassiz, 1865 is a successful, recent, macroplanktonic invader in European coastal waters, including theDutch Wadden Sea. It occurs year-round in substantial numbers. The effect of M. leidyi on structure of the gelatinous zooplankton community inthe Dutch Wadden Sea was studied by comparing data from before and after its introduction. Gelatinous zooplankton species composition in thewestern Wadden Sea was largely the same between 1980–1983 and 2009–2012. Only the hydromedusa Eutonina indicans (Romanes, 1876) wasabsent in recent years while Cosmetira pilosella Forbes, 1848 and Margelopsis haeckelii Hartlaub, 1897 were rare species observed only in2009–2012. M. leidyi was present since 2006, with high densities every year in summer and autumn. Predation pressure by scyphomedusae,ctenophores, and hydromedusae on fish larvae and zooplankton in 1980–1983 was low because of low densities and a temporal mismatch betweengelatinous zooplankton and their prey. Despite the decrease in abundance of scyphomedusae in recent decades, the introduction of M. leidyi and itslarge annual blooms have increased the overall importance of gelatinous zooplankton as predators in the western Wadden Sea, whereby M. leidyi isresponsible for most of the predation pressure on mesozooplankton in the area. Rather than outcompeting local species, M. leidyi appeared to fill ina previously unoccupied niche in the pelagic western Dutch Wadden Sea in late summer and autumn.