Thailand of tropical monsoon and savanna climates faces floods every year somewhere in the country and thus had their own traditional ways of coping strategies, especially in terms of architectural style and community building. However, floods, which had been traditionally a part of daily life in Thailand, began to be recognized as disasters in recent years, and the case of recent annual floods from the Sai river of Mae Sai District, Chiang Rai Province is not an exception. The northernmost city of Mae Sai District, Thailand, and its neighboring town Tachileik Township in Myanmar had rapidly developed and urbanized since around 1990 as border cities thanks to trade and tourism. The border markets on both sides around the border checkpoint have been the most popular tourist spots and have always been very crowded with small shops, vendors, and tourists. However, there is a transboundary river, called the Sai-Ruak river, between Mae Sai and Tachileik and it sometimes causes heavy floods to the border market areas almost every year during the rainy season in these 10 years although the scales of the floods and damages differ every year. With a socio-anthropological approach, this paper firstly argues the rapid urbanization of the riparian communities of the Sai-Ruak river as the cause of the recent floods in comparison with the rural border area in the past as well as the traditional resilient community building in northern Thailand. The rapid economic development without a good disaster management plan made the community vulnerable to the disaster. It secondly shows the difficulties and challenges of the flood management of the transboundary river, which requires the cross-border corporation and thus is different from the domestic river management. By doing so, it thinks about the way to build community resilience in the border city of Mae Sai District, Thailand in the future.