The 2023 warm-season months (May–September) in the United States experienced unprecedented heat waves, leading to a significant increase in emergency department (ED) visits for heat-related illness (HRI). CDC analyzed data from the National Syndromic Surveillance Program (NSSP) to compare HRI ED visit rates during 2023 with those from 2018–2022. The results showed that HRI ED visit rates peaked in several regions and remained elevated for prolonged periods. Males, especially those aged 18–64, were more likely to seek ED care for HRI. Regional differences were observed, with Region 6 (Arkansas, Louisiana, New Mexico, Oklahoma, and Texas) having the highest rates, while Region 2 (New Jersey and New York) had the lowest. The high temperatures in 2023 were more than three times higher than those in 2018–2022, and some regions experienced HRI ED visit rates exceeding the 95th percentile for multiple consecutive days. CDC issued public health alerts to address the increased demand for medical services and to guide public health interventions. The findings highlight the need for continued monitoring and preparedness to manage the public health impacts of extreme heat as climate change continues to intensify.The 2023 warm-season months (May–September) in the United States experienced unprecedented heat waves, leading to a significant increase in emergency department (ED) visits for heat-related illness (HRI). CDC analyzed data from the National Syndromic Surveillance Program (NSSP) to compare HRI ED visit rates during 2023 with those from 2018–2022. The results showed that HRI ED visit rates peaked in several regions and remained elevated for prolonged periods. Males, especially those aged 18–64, were more likely to seek ED care for HRI. Regional differences were observed, with Region 6 (Arkansas, Louisiana, New Mexico, Oklahoma, and Texas) having the highest rates, while Region 2 (New Jersey and New York) had the lowest. The high temperatures in 2023 were more than three times higher than those in 2018–2022, and some regions experienced HRI ED visit rates exceeding the 95th percentile for multiple consecutive days. CDC issued public health alerts to address the increased demand for medical services and to guide public health interventions. The findings highlight the need for continued monitoring and preparedness to manage the public health impacts of extreme heat as climate change continues to intensify.