Today’s world faces severe climate challenges, and there is a pressing need for genuine environmental advocacy within organizations and policymaking. This dissertation investigates the complex decision-making processes in the workforce and environmental communication and behaviour in organizations. The goal is to provide insights on stimulating environmental advocacy and enabling better-targeted behavioural change interventions. The thesis encompasses four independent but related papers that cover interdisciplinary research on environmental advocacy, examining the social backdrop of contemporary corporate sustainability, environmental communication, and strategies to encourage workplace environmentalism. The studies draw primarily from signalling and legitimacy theory while extending understanding of identity theory, power, and knowledge spillover. Overall, this dissertation contributes to a greater understanding of corporate sustainability by emphasizing the role of legitimacy in environmental leadership and sustainability communication. It fosters a more integrated, systematic, and comprehensive understanding of climate change communication in organisations, likely increasing the saliency of behavioural research in the mitigation debate and supporting evidence-based public policy.