To understand long-term motivations for second language (L2) learning/socialisation, this study analyses the biographical narratives of 30 Hong Kong Chinese who are proficient in Japanese by using ethnographic interviews. From a situated learning perspective, our research findings originate in the participants’ transformational episodes and centre on the following three dimensions of legitimate peripheral participation (LPP): (1) gaining legitimate access to L2 socialisation in Japanese-speaking communities of practice, (2) evolving processes from LPP to fuller participation in Japanese-speaking communities, and (3) constructing identities as L2 users of Japanese. Participants gained legitimate access to L2 socialisation relatively easily in Hong Kong, where interests in Japanese popular culture tended to initiate motivation. This accessibility increased owing to the internet, particularly online social networking services, leading to ongoing motivation development. Due to perceived cultural homologies between China and Japan, participants’ motivations were found to be fuelled by a sense of ownership regarding social ethics and other aspects of Japanese culture as part of their cultural identities as Chinese. The study concludes with useful implications for both classroom teaching and future research on long-term L2 motivations.