This paper analyzes the role of social movements in the creation and evolution of a welfare state in South Korea. We begin with a theoretical overview of the existing works on policy change, highlighting concepts such as ideas, policy paradigms, and windows of opportunity. We then examine state institutions, hegemonic policy paradigms, and the specific dynamics of welfare policy-making during South Korea’s authoritarian period (1961–1987). Next, characterizing the democratic transition in 1987 and the economic crisis of 1997–1998 as “windows of opportunity,” we probe how social movements emerged as “policy entrepreneurs” and played crucial roles in building welfare institutions and promoting welfare policies. In particular, we focus on the role of social movements in legislating the National Basic Livelihood Security Act in 1999 and consolidating fragmented health insurance systems in 2000. We conclude that social movements in democratized South Korea have assumed the role of policy entrepreneurs, filling the vacuum left by the central government and elite bureaucrats in the field of social welfare policy-making.