This article presents a summary review of the voluntary sector literature on third sector organizations (TSOs) and climate change. As governments around the world fail to respond adequately to the climate crisis, TSOs are called upon to work with governments, advocate for policy change, and support communities. However, the literature on TSOs and climate change remains limited. We reviewed 68 articles about TSOs and climate change and identified four mechanisms for voluntary action on climate change, which vary on two dimensions—focusing on advocacy or implementation and occurring within or outside government—and include policy advocacy, advocacy for behavior change, participation in governance, and direct interventions. We conclude with five key areas for future research: explaining the relationship between advocacy strategies and context, foregrounding the role of TSOs in climate governance, exploring direct interventions by TSOs, examining community-based TSOs, and linking TSO action and climate outcomes.