Current literature on relationship management has focused on formal mechanisms such as contracts but has neglected the inter-organizational behavior that determines the effective adoption of relational mechanisms in the context of megaprojects. This study examines the antecedents and consequences of relational behavior of participating organizations in megaprojects based on the theory of planned behavior (TPB). A sample of 285 senior managers working on China's megaprojects was surveyed, and the results were analyzed using partial least square structural equation modeling (PLS-SEM), revealing that: (1) the adoption of relational behavior is equally motivated by an organization's behavioral intention and perceived behavioral control (PBC), and (2) relational behavior, especially information exchange, contributes the most toward achieving a high project relationship quality. A closer examination of the data found that two moderating effects, namely project culture and prior experience with other participating organizations, were particularly important. These findings contributed to four implications for project participating organizations to cultivate relational behavior and improve relationship quality in megaprojects, including identifying potential behavioral benefits, enhancing relational competence, establishing collaborative culture, and reinforcing information sharing.