This study examines a case of union revitalisation based on business unionism, contrasting with the prevailing notion that union revitalisation occurs primarily through social movement unionism. The Korean Federation of Teachers Union (KFTU) was formed in 2017 to challenge the established Korean Teachers and Education Workers Union (KTU), which we describe as an example of social movement unionism. Criticising the centralised organisational structure and radical political struggles of the KTU, the KFTU has rapidly expanded by absorbing a substantial number of teachers in their 20s and 30s. In 2022, the relatively new KFTU claimed that it surpassed the KTU in membership. This study investigates how the newly-formed union, based on business unionism, emerged and flourished quickly. To address this inquiry, this research analyses three critical factors that influence union revitalisation: context (social and political changes), agents (a new generation of teachers) and strategies (organisational strategies of the KFTU). We find that these three factors intricately interacted with one another, leading to the rapid success of business unionism, but at the same time, the union's long-term sustainability remains in question.