This study aims to understand the factors that shape and construct the ‘illegality’ of migrants. Through the case study of international migration from Thailand to Korea, the study examines state policies and institutional and social factors that have constructed and sustained the illegal migration of Thai workers in the Korean context. The study underlines that illegal migration is neither random nor self-generating, but is constructed and sustained by the complexity of individual actions, institutional structures and migrant networks. In particular, various migrant institutions, which are increasingly operated on social media, are one of the crucial factors facilitating the illegal migration of Thai rural workers to Korea. The study demonstrates that in everyday life illegality is irrelevant. Migrants, employers, and locals seek economic benefits regardless of legal status. The study argues that one should avoid oversimplified views on undocumented migrants as victims of exploitation or ‘criminals’. Illegality is essentially the product of both legal-institution frameworks and dominant power in society.