Despite their lack of a formal status as subjects of international law, cities have been performing a relevant activity in the context of global climate governance. In fact, coping with climate change requires action from all levels of governance. The vulnerability of cities to climate change and its effects, hand in hand with their contribution to global GHG emissions, explain why cities have been particularly active in testing new rules, standards, and practices, which might be later codified as a treaty-based or a domestic statutory rule. Furthermore, cities have been pledging to comply with targets and deadlines of GHG emissions reduction, namely through local ordinances, if their Constitutions mandate cities to pursue environmental or climate goals. Cities’ pledges do not bind their States but sustain their States’ international commitments and help complying with the goals and obligations under the UNFCCC and the Paris Agreement. Finally, cities can participate as observers in the meetings of the Conference of the Parties to the UNFCCC and the Paris Agreement, where they can contribute to the shaping of treaty rules, and offer a decisive help in climate-related litigation before international bodies. The role of cities as global actors is short, but it has proved to be efficient and there is still potential for broadening their contribution to global climate governance.