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Extreme weather events are predicted to increase under climate change, with serious consequences for ecosystems and populations. The effect of these events on mating and fitness are largely unknown. We examined the effect of heat stress on mating activity and reproductive success of the barnacle Fistulobalanus albicostatus. In aquarium tidal tanks, barnacles were subjected to low tide body temperatures (heat stress treatment, 39 degrees C and extreme heat stress treatment, 45 degrees C) based on field data from temperature biomimetic loggers, including on days of extreme heat. Barnacles were filmed post heat stress during high tide and their larvae collected. Barnacles exposed to the heat stress treatment rarely attempted to mate (<1% of individuals displayed mating activity) compared to controls (nearly 30% of barnacles mated). Barnacles exposed to heat stress still had lowered mating activity after 1 wk of non-stressful conditions. Heat-stressed barnacles almost never released larvae. In the extreme heat stress treatment, barnacles had additional negative impacts, decreasing mating activity by nearly 1 order of magnitude over a period of 3 wk. In conclusion, increases in heat stress under climate change have the potential to restrict mating with potential consequences for fitness. Our results provide evidence that extreme heat events can disproportionately affect mating behaviour.