The construction industry in Hong Kong has adopted relational contracting (RC) as a way forward to address frequent conflicts in construction projects and to promote sustainable development. Despite this effort, adversarial behavior of project team members is still prevalent, stemming from conflicting agendas, which hinders the successful implementation of RC. There is a need to improve collaborative attitudes and behavior among project team members in RC projects, but there is still a lack of understanding of factors that can promote this inter-organizational collaboration. Therefore, using the theory of planned behavior, this research investigates factors that form relational attitudes, collaborative intentions, and collaborative behavior, and their relationships in RC projects in Hong Kong. Quantitative data were collected from experienced practitioners in RC projects and were analyzed using structural equation modeling (SEM). The results reveal that senior management commitment and relational norms are needed to nurture relational attitudes, which in turn influence the development of collaborative intentions. These intentions can eventually promote collaborative behavior, which is expressed by teamwork, affective trust, and extra-role behavior (striving beyond roles to maintain collaboration). The findings advance knowledge and contribute to practice by providing a structured process to nurture collaboration in RC projects for sustainable development.