Climate change is predicted to increase the frequency of potentially hazardous heat conditions across the United States, putting the incarcerated population of 2 million at risk for heat-related health conditions. We evaluate the exposure to potentially hazardous heat for 4,078 continental US carceral facilities during 1982–2020. Results show that the number of hot days per year increased during 1982–2020 for 1,739 carceral facilities, primarily located in the southern United States. State-run carceral facilities in Texas and Florida accounted for 52% of total exposure, despite holding 12% of all incarcerated people. This highlights the urgency for enhanced infrastructure, health system interventions and treatment of incarcerated people, especially under climate change.