A brisk building boom of megaprojects leads to a wide range of environmental problems, particularly in developing countries such as China. To prevent environmental problems effectively, megaprojects require proactive environmental initiatives based on individual, voluntary, and discretionary behaviors, which are also known as organizational citizenship behaviors for the environment (OCBEs). The OCBEs (e.g., sharing knowledge to prevent pollution, making suggestions to minimize waste) play an important role in improving megaproject environmental performance. However, this line of research is still in its infancy and the institutional-psychological mechanism leading megaproject practitioners to engage in OCBEs is largely unexplored. To fill this gap, this paper presents an individual-level analysis that investigates how institutional pressures impact project practitioners’ OCBEs according to the survey data collected from China’s megaprojects. The results obtained by partial least-squares analysis indicate that both mimetic and normative pressures have a significant impact on OCBEs, and such relationships are partially mediated by organizational support. Nevertheless, this paper did not find evidence for a significant impact from coercive pressures. These findings provide a new insight into making use of institutional forces to stimulate the emergence of OCBEs, improving the environmental performance in megaprojects.