Anthropogenic environmental changes continue to threaten species globally. For example, translocation of species has caused unintentional hybridisation, which has contributed to species declines. On the other hand, hybridisation can be used to increase the evolutionary potential of species vulnerable to rapid environmental change, although the benefits of mixing genetically divergent lineages do not come without risks to individual fitness and the long-term viability of populations. Here, we use a combination of genome-wide Single Nucleotide Polymorphism (SNP) markers, mitochondrial DNA sequencing and measurements of growth rate to determine the genetic consequences of hybridisation between two congeneric marine gastropods across 27 years (~18 generations). Multigeneration hybridisation resulted from the introduction of the intertidal periwinkle Bembicium vittatum (a direct developer) into the native range of its congener Bembicium auratum (a species with planktotrophic larval dispersal). Despite significant genetic divergence between the species, we found no direct evidence of outbreeding depression in the admixed population. Instead, we found evidence for heterosis, which dissipated over time. After an initial lag, the frequency of introduced B. vittatum alleles declined dramatically in the hybrid population. However, a few B. vittatum alleles (3.18%) increased significantly in frequency against the overall trend, providing evidence of adaptive introgression. In the context of hybridisation as a conservation management tool, our results provide some evidence of the potential benefits that can be gained, and suggest that the costs due to outbreeding depression can be small.