Students' social identity is considered to have profound effects on their learning and academic performance, but there was a lack of research on whether students' social identity would also influence their aspirations to pursue STEM careers. This research studied the associations between the social identity of students and their aspirations to engage in STEM careers. Nearly 4,000 Hong Kong students from three key learning stages participated in a survey related to their STEM aspirations. Data analysis using multiple regression and structural equation modelling showed that local and national identity were predictive of STEM career aspirations. Moreover, students' sense of contribution to the nation and local society, as well as the perceived value of STEM professionals were mediators of the impact of their social identity on their STEM career aspirations. The implications provide insights for policymakers and educators to formulate appropriate curricula benefiting students' social identity development as well as their STEM aspirations.