This chapter focuses on how boundary spanning can lead to transformative climate actions. It investigates the conditional factors that support knowledge co-production through spanning various boundaries, with the view to developing the collaborative research and long-term partnerships across science–policy–practice boundaries, one of the cornerstones of the Global Research and Action Agenda on Cities and Climate Change Science (GRAA). New modes of practice in planning and designing cities for people and non-human species are needed to respond to climate change, biodiversity extinction, and sustainable development goals. The complexity inherent in socio-ecological systems in cities requires integrated and sustained engagement between scientists, policymakers, and practitioners. Boundary work theory provides a useful lens for identifying connections among science, policy, and climate actions through transdisciplinary mediation, translation, and communication of knowledge. Boundary spanners undertake cross-boundary work to disrupt status quo practices, challenge norms, and collaborate across organizational, sectoral, and disciplinary boundaries. The critical role of “boundary spanners” (or boundary actors) has been increasingly acknowledged in different disciplines, including health and management, but to a much lesser extent in the built-environment disciplines. This chapter identifies key boundary spanning roles in selected practices and projects. It reveals how boundary actors span different types of boundaries in climate action, and identifies a spectrum of influencing factors that support or hinder an enabling ecosystem for boundary spanning. The findings highlight the diversity of roles and modes of practices, but also the need for social innovation in supporting relational dimensions and interactional expertise for enabling systemic change towards collective climate actions in cities.