This comparative analysis aims to capture the complex roles and positionings of the International Baccalaureate (IB) in conjunction with local education systems in Hong Kong, Singapore, and South Korea. Our analysis focused on how the IB's institutional legitimacy is presented in the three societies. We conducted a documentary analysis of texts on the introduction and implementation of IB programmes into local school systems. Our findings suggest that there are commonalities and variations in how the IB is interpreted by key local agents and is positioned into local education systems. Specifically, across the three societies, the IB has expanded continuously. At the same time, its institutionalisation process varies by each society's socio-historical context and needs: substantive legitimacy as the international curriculum of choice in Hong Kong, a quiet supplement to elite education in Singapore, and instrumental curriculum borrowing for fixing the education system in Korea. We also find that the institutionalisation of the IB is limited at a symbolic level and controlled by the Singaporean government, while the IB is saliently promoted by local education authorities in the context of education reform in Korea. The institutionalisation process of the IB in Hong Kong is primarily swayed by market principles under the existing school choice system.