This article investigates the ecological components and implications of cosmopolitan citizenship. Supporters of the model of cosmopolitan citizenship tend to take the natural environment as a background assumption. In contrast, this article argues that if they want to fulfill the demands of cosmopolitan justice, cosmopolitan citizens should also be ecological citizens. The first part highlights that cosmopolitan citizenship relies on two ecological claims: the environmental impacts claim and the environmental precondition claim. The second part addresses two tensions between cosmopolitan citizenship and ecological citizenship: the tension between economic development and environmental protection and the tension between resourcism and strong environmental sustainability. The third part explores two implications of the convergence between ecological citizenship and cosmopolitan citizenship: the interconnection of the public and the private spheres and the move from anthropocentrism to nonanthropocentrism. The objective is to link cosmopolitan theory with green political theory by taking a fresh look at cosmopolitan citizenship.