Development of basin-scale rainfall-runoff modeling systems is essential for integrated water resources management in terms of both assessing management alternatives and real-time resource management. This study developed a distributed rainfall-runoff modeling system based on the Object-oriented Hydrologic Modeling System (OHyMoS) for a large river basin (the Daechung Dam basin, South Korea; 3,994 km(2)) that is highly regulated by multipurpose dams. We applied three hydrologic element modules to simulate hillslope runoff, channel runoff, and dam reservoir outflow and then linked these modules together under OHyMoS to simultaneously predict discharges at eight specific outlets. This newly developed rainfall-runoff modeling system generally provided acceptable hydrographs during the typical Korean rainy period from 1 June to 30 September, although the simulated hydrographs for extreme flood events during typhoons and local heavy rainfall were underestimated compared to measured hydrographs. The developed modeling system can be used for water resources planning and management in the Daechung Dam basin and also readily extended to other large basins, such as the whole Geum River basin (9,835 km(2)) that includes the Daechung Dam basin, by incorporating sub-basin models into the system.