This study aimed to examine the role of positive relationships and academic achievement in predicting student well-being, i.e. subjective happiness. This study employed a longitudinal design with two waves of data collection on a sample of 786 primary school students in Hong Kong. Students completed questionnaires on parent-child relationship, teacher-student relationship, peer relationship, academic achievement, and happiness at the beginning and end of the school year. Path analysis was used for data analysis. The results indicated that Time 1 parent-child relationship and peer relationship were associated with Time 2 academic achievement, after controlling for Time 1 academic achievement. However, only Time 1 parent-child relationship was predictive of Time 2 happiness. Time 2 academic achievement was a mediator between Time 1 parent-child relationship and peer relationship and Time 2 happiness. Furthermore, girls reported higher levels of academic achievement when they perceived better peer relationship than did boys, and girls were happier when they had higher levels of academic achievement than were boys. The results suggested the need to put in place strategies to enhance parent-child relationship, peer relationship, and a harmonious classroom.