The non-indigenous tanaidacean crustacean Sinelobus vanhaareni Bamber, 2014 was first found in northern Europe in 2006 and has since spread to the northern Baltic Sea. Here, we surveyed the distribution of the species in different habitats in southwestern Finland, focusing on vegetated macroalgal and seagrass habitats (i.e., Fucus vesiculosus beds and Zostera marina meadows). We also evaluated its potential impacts by synthesizing current knowledge on the traits and ecology of the species, and identified knowledge gaps. We found that S. vanhaareni is now present throughout most of the southwestern Finnish coast, in a number of vegetated and non-vegetated substrates down to 25 m depth. Furthermore, the presence of egg-brooding females in most areas also confirms that the population is self-sustaining. The species is especially abundant in shallow macroalgal belts and eelgrass meadows, which are critical habitats for biodiversity, ecosystem functioning, and ecosystem service provisioning, highlighting the need to understand the effects of S. vanhaareni in these important ecosystems. Its presence on boat hulls and in marinas and harbours suggests that recreational boating may be a major spread vector, while drifting macroalgal fragments may also contribute to regional spread. At this stage of invasion, we found high overlap in epifaunal community composition in sites where S. vanhaareni was present and sites where it was absent. Based on the functional traits of S. vanhaareni and closely related species, we infer that it is likely part of the detritus-based pathway in benthic food webs. However, additional sampling and experiments are necessary to determine the true extent of its distribution and to quantify trophic links (through stable isotope analysis, gut content analysis, and experimental trials) to fully understand its effects on communities and trophic networks in the northern Baltic Sea.