This article examines how unions build worker power for day laborers in South Korea’s construction industry in the context of widespread informality. Drawing upon regional case studies of the Korean Construction Workers Union (KCWU), we find that construction day laborers experience poor working conditions and rampant employment violations under multiple layers of subcontracting that enable capital to bypass existing labor laws and regulations. Despite the regulatory challenges of complex subcontracting systems, unions can still exert direct pressure on firms to improve informal working conditions by securing and enforcing creative collective agreements. Key to this process is the development of regionally-specific forms of worker power that target firms located higher up the subcontracting chain to take responsibility for informal working conditions. Although the scope of influence varies depending on the type of worker power that unions cultivate (e.g. structural, associational, and symbolic), each form of worker power has enabled unions in different regional contexts to establish uniform standards regarding job quality and job security despite formal restrictions on the legal authority of unions as bargaining agents for informal workers. While such approaches require a high level of organizational and strategic capacity, they demonstrate the ongoing relevance of unions in challenging the global turn to informal work through workplace organizing and collective bargaining.