Drawing on the job demands-resources theory, this study pioneered the investigation of the influence of academic psychological capital (including self-efficacy, hope, resilience, and optimism) on PhD students' creativity (including novelty and usefulness). It further explored the mediating role of academic engagement (including vigour, dedication, and absorption) in this relationship. A sample of 376 PhD students at two comprehensive universities in Jiangsu province, mainland China, responded to an online survey. Results showed that: (1) PhD students' self-efficacy, resilience, and optimism positively predicted novelty, and all four dimensions of academic psychological capital positively predicted the usefulness dimension of creativity; and (2) dedication mediated the effects of hope and optimism on both novelty and usefulness; however, vigour and absorption did not mediate the association between academic psychological capital and creativity. The findings highlight the significance of academic psychological capital and academic engagement, especially dedication, in optimising PhD students' creativity.