This thesis presents a republican account of the legitimacy of border control. It thus contributes to two debates in contemporary political theory: first, the debate over whether contemporary border controls satisfy applicable criteria of normative legitimacy and, second, the debate over what the republican commitment to the ideal of non-domination entails for the border regime of a defensible republic. The thesis offers a critical discussion of the theoretical debate on immigration and border control, which has in large part focused on the question of whether states are in the possession of the so-called right to exclude. It argues that identifying the relevant legitimacy standard for border control is pressing in light of recent changes to the border regimes of the world's most powerful states. These regimes are externalised and coordinated in ways that allow them to exercise power over vulnerable migrants without incurring responsibilities for the protection of their human rights. The contemporary practice of border control is characterised by unaccountable power, posing a challenge to the legitimacy of the implicated regimes. The thesis then argues that the neo-republican concept of domination convincingly captures the specific wrong involved in the contemporary practice of border control, and that its converse ideal of non-domination can serve as a plausible legitimacy standard for border control. Under contemporary practice, a set of wealthy states exercise arbitrary power over a set of vulnerable individuals, resulting in relations of domination that suffice for illegitimacy. This proposal is defended, first, from the philosophical objection that the concept of domination is radically overinclusive and that the ideal of non-domination therefore cannot serve as a legitimacy standard. Second, the thesis refutes the objection that any attempt to rectify the domination of migrants through supranational institutions will undercut states' capacity for self-determination and thus inevitably lead to more domination.