Prior research on the development of the welfare state in East Asia has mainly focused on domestic socioeconomic and cultural factors. This study analyzes the formation process of the Korean welfare state with special attention to global policy diffusion. Case studies on the two main areas of social policy, old age security and social assistance, show that recent welfare reforms in Korea have led to the emergence of a welfare state in terms of three criteria: state goals, principles of social provision, and universalism. This study demonstrates that the diffusion of welfare internationalism played an important role in the development of the welfare state in Korea, but also that policy diffusion produced differential welfare outcomes, depending on the relationship between state and society in varying phases. In the era of the developmental state, the principles and systems of the welfare state were formally and legally imported, but were not put into practice ("decoupling"). However, they were progressively implemented in the era of the people's government ("coupling"). This study demonstrates that combining domestic and international factors can provide a fuller understanding of welfare state formation in a transitional society such as South Korea.