As the threat of climate change and associated heatwaves grows, we need to understand how natural populations will respond. Inter-generational non-genetic inheritance may play a key role in rapid adaptation, but whether such mechanisms are truly adaptive and sufficient to protect wild populations is unclear. The contribution of paternal effects in particular is not fully understood, even though the male reproductive system may be highly sensitive to heatwaves. We used the zebrafish Danio rerio to investigate the effects of heatwaves on male fertility and assess potential adaptive benefits to their offspring in a number of large-scale heatwave experiments. Heatwave conditions had negative effects on male fertility by reducing gamete quality and fertilisation success, and we found indications of an adaptive effect on hatching in offspring produced by heatwave-exposed males. Our findings highlight the importance of including male and female fertility when determining species ability to cope with extreme conditions and suggest that parental effects provide limited adaptive benefits.