Indigenous people have participated in United Nations climate change conferences for over 30 years under informal conditions. Their formal opportunity to voice concerns and share traditional knowledge emerged  when the Local Communities and Indigenous Peoples Platform became operational in 2018 under the United Nations Framework Convention on Climate Change. Despite the growing and formalised participation of Indigenous people, their demands and demonstrations for justice suggest they tend not to perceive global adaptation governance institutions as legitimate. Low or no perceived legitimacy could hinder cooperation between Indigenous people and global adaptation governance institutions, thereby potentially undermining effective and fair climate adaptation. Such societal implications underscore the need to deepen our understanding of the drivers shaping Indigenous people's legitimacy perceptions of United Nations climate governance institutions, since solid knowledge on this topic is limited. Hence, this thesis develops a theory of justice beliefs as drivers of legitimacy perceptions, drawing on and advancing studies of global adaptation governance, sociological legitimacy, and (Indigenous) environmental justice. Employing a mixed-methods approach, this thesis combines quantitative and qualitative methods to provide both general and in-depth insights. Its primary contribution is to advance the theoretical and empirical understanding of recognitional, procedural, distributive and transitional justice factors as perceived legitimacy drivers. Moreover, the analysis offers preliminary findings on the role of participation in shaping legitimacy perceptions. The thesis discusses implications for the study and practice of global adaptation governance in particular and for Indigenous studies in International Relations more broadly.