Education for sustainable development (ESD) has for decades been a focus internationally and has been embraced in the Japanese curriculum since 2008. Given UNESCO's emphasis on teacher training for ESD and well-established in-service teacher training programmes in Japan, the aim of this study is to explore whether ESD teacher training could help Japanese in-service teachers to implement ESD in school. Semi-structured individual interviews with 12 in-service teachers from primary to lower secondary schools were conducted via Zoom due to the COVID-19 pandemic. It was found that in-service teacher training for ESD had a positive impact on transforming the teachers in both their professional and personal lives. However, challenges remain regarding the promotion of ESD in schools because of lack of common ground with other teaching colleagues and time constrains. The implications of implementing global ESD in schools are discussed.