Vibrio spp. are ubiquitous in aquatic environments. In temperate regions, including the Baltic Sea, these bacteria, which include facultative pathogenic species, proliferate during warm water periods and may thus pose a risk to human health. However, while present at relatively high abundances in the summer months, Vibrio spp. are rarely detected during cold water periods. Several potential winter reservoirs, such as fish and sediment, have been discussed, but the seeding sources that give rise to Vibrio during the summer months are unknown. In this study, we employed a combination of cultivation-independent genus-specific digital droplet PCR, Illumina 16S rRNA gene sequencing, culture-dependent determinations of colony-forming units (CFU) and genus-specific strain isolation to identify possible winter reservoirs of Vibrio spp. Using these techniques we demonstrate that in the southern Baltic Sea coast the invasive species round goby (Neogobius melanostomus) serves as a winter reservoir for goby-specific Vibrio assemblages, including facultative pathogenic V. vulnificus. In areas with higher individual densities, the amount of Vibrio spp. cells carried by the goby is comparable to that found in equal volumes of water and sediment but only 1/1000th of the spatial share in the overall coastal environment. Thus, by providing a winter reservoir for distinct Vibrio communities, areas in which round goby densities are high may be starting points for summer blooms of pathogenic Vibrio species.