Heat waves pose critical health risks for unhoused people. However, little is known about how this population perceives and copes with heat, particularly in Europe. Employing a sequential mixed-methods approach, we investigated dimensions of unhoused individuals’ heat perceptions, coping behaviors, and barriers via semi-structured interviews (N = 9) to inform the design of a quantitative survey administered across 13 German cities in 2024 (N = 252). Results revealed that many homeless people experience fatigue, nausea, or faintness on hot days, whereas only half of respondents considered heat-related health problems likely or dangerous. Protection was most strongly associated with perceiving that people important to participants protect themselves. At the same time, protective behavior was structurally constrained by limited opportunity, including inadequate access to hydration and cooling spaces, eviction, and lack of cool and secure places to store medication. Overall, the results warrant urgent policy interventions, such as improving access to hydration resources, establishing dedicated cooling shelters, and suspending evictions from cool places in the case of official heat warnings. Furthermore, information-based interventions should raise awareness regarding heat-related health risks and protective actions among unhoused populations, stakeholders working with them, and the general public.