Extreme weather events, changes in rainfall patterns, and sea-level rise due to climate change increases the risk of flooding. Cities, in particular, are highly vulnerable to flood risks as they are centers of socioeconomic activity and host large populations. Flood vulnerability assessment is a method that combines social and physical indicators and can assist cities in recognizing the potential threats posed by the consequences of climate change. Istanbul, the most populous city and the economic, commercial, and social hub of Turkey, was selected as the study area due to its long history of significant flood disasters. The assessment method was applied to the Kaynarca River Basin in the Pendik district of Istanbul. In this study, flood vulnerability was evaluated using multi-criteria assessment principles by assigning different weights to various indicators. Within the scope of the study, nine indicators related to climatological, demographic, economic, and physical factors were quantified and used to formulate a flood vulnerability assessment index. The index was then weighted using the Analytical Hierarchy Process (AHP) method, incorporating input from a diverse group of stakeholders, experts, and policymakers, thereby ensuring a more inclusive and participatory assessment process. The main outcome of this study is to provide policymakers with an up-to-date and comprehensive analysis of flood vulnerability, which can serve as a foundation for informed decision-making. Mapping flood vulnerability is essential for making cities more resilient to catastrophic events and the impacts of climate change, and for helping municipalities optimize the allocation of their resources.