Megaproject stakeholders sometimes go beyond the requirements of their contract and perform actions that benefit others, despite conflicting with their own interests. This type of organizational citizen behavior (OCB) is frequently seen in the context of construction megaprojects, but its characteristics and drivers are largely unknown and little studied. Using a quantitative cross-case study, this paper presents an overview of OCB practices in construction megaprojects, which will be referred to as megaproject citizenship behavior (MCB). The paper proposes a framework for MCB based on structured interviews with megaproject participants. This framework is composed of five types of behaviors: compliance, contingent collaboration, harmonious relationship maintenance, conscientiousness, and initiative. Furthermore, MCB demonstrates three distinct features beyond the aforementioned behaviors: actions based in MCB extend beyond the megaproject organization's boundaries toward all stakeholders, they occur at an interorganizational level, and they promote harmony among participating organizations. This study's findings are discussed based on their implications for theory and future research in megaprojects. © 2018 American Society of Civil Engineers.