Understanding Legitimacy Construction Through Online Community Discourse in Megaprojects: evidence from the Lyon–Turin megaproject

• Online community sentiment toward megaprojects is highly event-sensitive and evolves dynamically over time. • Protest events act as powerful amplifiers of legitimacy challenges, strengthening opposition narratives. • Legitimacy dynamics exhibit both intergroup polarization and intragroup fragmentation within online communities. • Negative project sentiment spills over to the promoting organization, weakening organizational legitimacy. • Digital arenas have become critical spaces where project legitimacy is continuously constructed and contested. Legitimacy is a critical condition for the viability of megaprojects, yet little is known about how it is constructed and contested within digital arenas. This study examines the role of online communities in shaping project legitimacy, drawing on the case of the transnational Lyon–Turin high-speed rail project. Using a longitudinal dataset of over 190,000 tweets collected over a 15-year period, we analyze how project-related events, organizational communication, sentiment, engagement, and polarization interact over time. The results show three main findings. First, online legitimacy is highly event-driven: governance, economic, and protest-related events generate statistically significant shifts in community sentiment, with public protests producing the strongest and most consistent effects. Second, online communities exhibit pronounced internal heterogeneity. Rather than responding uniformly, both supporters and opponents display intragroup polarization, revealing fractures and divergent interpretations of the same events. Third, increases in negative sentiment toward the project are associated with reduced engagement with the project-promoting organization, indicating that online disaffection can spill over and undermine organizational legitimacy. This study advances a process-oriented understanding of project legitimacy as contested, fragile, and temporally sensitive. The findings highlight the strategic importance for project-promoting organizations of systematically monitoring online discourse and engaging proactively with digital publics throughout the project lifecycle.