Abstract Inequality, as well as the scope of the duty of justice to reduce it, has always been a central concern of political justice. Income taxation has been seen as a key tool for redistribution and the state was the arena for discussions of justice. Globalization and the tax competition it fosters among states change the context for the discussion of distributive justice. Given the state’s fading coercive power in taxation and the decreasing power of its citizenry to co-author its collective will due to global competition, we can no longer assume that justice can be realized within the parameters of the state. International tax policy in an effort to retain justice often opts for cooperation as a vehicle to support distributive justice. But cooperation among states is more than a way for them to promote their aims through bargaining. Rather, it is a way for states to regain legitimacy by sustaining their very ability to ensure the collective action of their citizens and to treat them with equal respect and concern. The traditional discussion in international taxation seems to endorse a statist position — implicitly assuming that when states bargain for a multilateral deal, justice is completely mediated by the agreement of the states. In contrast, this Article argues that such a multilateral regime intended to provide the state with fundamental legitimacy requires independent justification. Contrary to the conventional statist position, I maintain that cooperating states have a duty to ensure that the constituents of all cooperating states are not treated unjustly because of the agreement. I argue that not only cosmopolitanism but political justice too requires that a justifiable cooperative regime must improve (or at least not worsen) the welfare of the least well-off citizens in all cooperating states. I explain that cooperation alone is no guarantee of improved welfare and that certain transfer payments between rich and poor countries might be required to ensure this.