Despite the growing interest in flood risk assessment in recent years, the relationship between flood vulnerability and income disparity remains underexplored, especially in the dimension of residential basements. To address this research gap, we first estimated annual income at building floor level using housing sale prices and associated conversion indices. The reverse geocoding technique was subsequently adopted to assign income data of residential basements to associated building footprints for further analysis. This study assessed the impacts of flood events between 2010 and 2022 on different income groups of basement residents in Seoul, a city characterized by global financial standing and high population density, but also high vulnerability to floods. Our findings reveal that (1) flood events disproportionately impact areas with lower housing sale prices, which are also home to a large proportion of lower-income residents; (2) 85.3 % of affected basement households belong to the below-middle-income group, with even higher proportions observed in lower-income districts ranked 10th or below; (3) four districts Gangseo, Guro, Songpa, and Gangnam exhibit a pattern in which higher-income households are generally less affected by flooding; and (4) in some sub-districts, below-middle-income basements are located in areas characterized by both high flood exposure and socio-economic vulnerability. These results suggest two key policy implications, including (1) below-middle-income basement residents, particularly those in highly vulnerable sub-districts, should be prioritized in flood mitigation strategies; and (2) flood risk information should be more effectively integrated into the housing market to better inform both prospective buyers and policymakers.