We live on an increasingly hot planet, with more frequent and intense heatwaves affecting populations worldwide. Evidence shows that extreme heat can take a toll on mental health, particularly among people living in dense urban areas, where temperatures tend to be highest. Urban nature, especially trees, but also grass, green roofs, and green walls, can help reduce temperatures in cities. But can urban nature also help mitigate the mental health impacts of extreme heat? To address this question, I will present findings from two datasets. The first is based on large-scale electronic health records collected in the UK over a 15-year period (2008-2022). The second comes from a citizen science project in which participants shared their experiences using a smartphone app (https://www.natureboostproject.com/). Together, these datasets help us understand how urban nature can support mental health in periods of very hot weather. This is especially evident in individuals with existing mental health conditions, who are most vulnerable to the adverse effects of extreme heat. Finally, I will refer to our team's efforts to translate these insights into policy action in the United Kingdom. Notably, the recent 10-Year Health Plan for England does not include a single reference to nature in its 168 pages. By failing to recognise the central role of nature in human health, governments are overlooking one of the simplest and most effective ways to build both human and ecological resilience.