Abstract Background Thermal indices assess individuals’ perception of their ambient thermal surroundings (thermal comfort and heat/cold stress) considering several meteorological factors, such as temperature, humidity, wind, and radiation. As climate change progresses, there is an increased demand for accurately monitoring the association between thermal environment and human health. This study summarizes the available evidence on the association between thermal indices and health outcomes. Methods We conducted a systematic search in PubMed, Scopus, and Web of Science until December 31st, 2023, to identify all relevant studies examining the association between thermal indices and health outcomes in outdoor environments. Results A total of 5,034 electronic records were screened. Of these, 310 were considered eligible, examining 1,143 associations. Seventeen indices were identified, with 32 (10.3%) articles using more than one index. The most commonly employed index was Apparent Temperature (articles: n = 141, 38.2%; associations: n = 634, 55.5%). The specific-cause health outcomes examined spanned 18 categories of the International Classification of Diseases. Most associations (n = 304, 26.6%) pertained to mortality/morbidity related to diseases of the circulatory system, followed by all-cause morbidity/mortality (n = 288, 25.2%). The 48.6% (n = 556) of the associations suggested an increased risk of adverse health outcomes when indices increased while 32.5% (n = 372) of the associations were non-significant. The 14.2% (n = 162) of the associations suggested an increased risk of adverse outcomes when indices decreased while 16% (n = 183) of the associations were non-significant. Conclusions The relevant literature revealed associations between thermal indices and health outcomes. Therefore, thermal indices, considering several meteorological variables, could be a useful tool for public health planning. However, there is diversity in their methodologies and reporting. Key messages • Summarizing the published evidence regarding thermal indices is expected to attract the interest of public health professionals and policy makers, and enhance their use. • Implementing integrated methods to monitor the association between thermal conditions and health, including using thermal indices, could be valuable for developing and evaluating policies.