This study adopted a single-group pretest-posttest design to explore the changes in 2,876 undergraduate students in positive youth development, psychological well-being and desired graduate attributes after they took a leadership subject utilising the positive youth development approach at one university in Hong Kong. The subject aims to promote undergraduates' leadership qualities, particularly their intra- and inter-personal competencies. Participants completed the same objective outcome evaluation forms before and after taking the subject. We found that the participants showed significant positive changes in most positive youth development attributes (i.e. self-determination, cognitive competence, behavioural competence, social competence, emotional competence, self-efficacy, spirituality, and clear and positive identity), life satisfaction and desired graduate attributes (including problem-solving ability, critical thinking, life-long learning and ethical leadership). The findings demonstrate that leadership subjects can promote the positive youth development of Chinese university students and enhance their psychosocial well-being and desired graduate attributes.