•This study frames heat injustice in aspects of cause-related, effect-related and process-related injustice.•Empirical survey was conducted in Yangtze River Delta and Chengdu-Chongqing urban agglomerations.•This study geographically revealed effect-related and process-related injustice in YRD and CCEC regions.•The cause-related, effect-related and process-related injustice depended on the socioeconomic factors.•Urban planning and design for heat-resilient cities and setting temporary shelters should be prioritised in heat actions. Urban heat has affected numerous cities with increasingly severe environmental, social, economic, and health impacts. It is urgent to develop plans and take actions to beat the heat before cities are locked into unintended consequences. An understanding of heat injustice is important to support heat plans and actions for accurately alleviating heat-related risks and threats, scientifically avoiding unexpected conflicts and disputes, and actively driving citizens to participate in actual implementation. Existing studies have primarily focused on effect-related injustice (e.g., morbidity and mortality), while a systematic understanding of heat injustice is lacking. To overcome this research gap, this study framed heat injustice in terms of cause-related, effect-related, and process-related injustice, and empirically verified them based on a questionnaire survey in the Yangtze River Delta (YRD) and Chengdu-Chongqing Economic Circle (CCEC) regions. The survey results from 2704 respondents indicated that effect-related injustice and process-related injustice were geographically prominent in the YRD and CCEC regions. Socioeconomically, cause-related, effect-related, and process-related injustice occurred with the identification of groups who thought the heat was more severe were more susceptible to physiological and psychological impacts, and had lower levels of heat-related awareness and knowledge. The results also indicated that both YRD and CCEC regions should urgently develop heat action plans, which should be regionalized and group-specific. Urban planning and design for heat-resilient cities and setting temporary cooling shelters should be prioritized. Moreover, this study identified inconsistencies among higher levels of heat-related knowledge, lower levels of cooling strategy familiarity, and lower urgency of heat actions among the elderly, less educated, unhealthy, and wealthy groups. Overall, this study provides a reference for understanding heat injustice and formulating unbiased and implementable heat action plans.