Heatwaves are becoming more common and impact health. We conducted a representative survey in June 2022 in Germany to determine people’s knowledge and protective behaviours on heat days. In data from 953 respondents, we found that a large proportion informed themselves about upcoming heat days, but there are considerable gaps in knowledge. While knowledge was not related to taking up protecting behaviour, other predictors were (e.g. risk perception). Health campaigns should therefore not only aim to improve knowledge but also address risk perceptions, facilitate social learning, communicate social norms and remove barriers that prevent protective behaviours.