Populations at range margins of marine organisms are at the front line of climate-induced changes in abiotic factors and are thus particularly susceptible. Especially, macroalgae that are externally reproducing rely on optimal environmental conditions for gamete release and subsequent fertilisation. However, the effect of climate change–forced decrease in salinity on these critical life stages has been largely overlooked. We tested the impact of forecasted hyposaline conditions on a marginal population of the rockweed Fucus vesiculosus growing at 5.8 PSU on the Finnish coast of the Baltic Sea. We incubated individuals with receptacles for at least seven days at 2.5, 4.1, 5.8, and 7.2 PSU and determined their gamete release and subsequent fertilisation success. We further tested sperm performance at 3.5, 5.0, and 6.1 PSU. Salinity of 2.5 PSU, which is predicted to occur in the region by the end of this century, reduced egg release. In contrast, sperm and antheridia release were not consistently affected by the different salinities, but the size of sperm swells at 3.5 PSU. Because fertilisation success was drastically reduced at 2.5 and 4.1 PSU, we suggest that sperm performance was compromised such that sperm dysfunction hampered fertilisation success. Our results demonstrate that the forecasted hyposalinity negatively affects egg release and sexual reproduction in F. vesiculosus at its northern distribution limit. This macroalga can probably only withstand the future decrease in salinity when populations proliferate via asexual reproduction.