Climate change increases the likelihood of heatwave events, causing human thermal discomfort and even mortality. However, it is not clear to what extent humans with long-term and short-term experience of hot-summer exposure can adapt to thermal comfort in urban public open spaces when both experience their heatwave periods. Therefore, this study aims to investigate outdoor perceived thermal comfort in urban public open spaces during heatwave periods between two groups of people who have long-term and short-term experience of hot-summer exposure. Field surveys were conducted in public squares and parks during the heatwaves in Chongqing, China and Reading, the UK. Chongqing is known as a ‘furnace city’ and people have been living in a hot summer for a long time, while in Reading the summer is warm and people unusually experience the heatwave. The main results show that Chongqing respondents living in a hot climate for a longer period can endure more heat than Reading respondents during the heatwaves, indicating that Chongqing respondents have more thermal resilience. Besides, different behavioural adaptation measures show that people are active participants to choose their thermal preferences, rather than passive recipients of the thermal environments. The research implication contributes that protective measures against heatwaves need to be taken for pedestrians, including more shaded places with efficient ventilation design for sheltering and handy facilities such as drinking fountains and water spray. The research has novelty in deepening the dynamic theory of human thermal comfort and providing empirical evidence of thermal adaptation in extreme-high temperature events.