This paper argues that recent debates on egalitarian objections to immigration restrictions overlook a crucial, powerful normative principle that underpins objections to inequalities: any inequalities between morally equal persons – whether in goods, resources, welfare but also in powers, statuses, rights, and freedoms – that arise from morally arbitrary factors are pro tanto unjust. This principle of moral arbitrariness is fundamental to both luck and relational egalitarianism yet is often missing from debates that apply such theories to migration ethics. The (...) result of this omission is that certain arguments that purportedly reject luck egalitarian cases for open borders in fact fail since they fail to recognise the normative force of the principle of moral arbitrariness; yet, simultaneously, relational egalitarian cases for open borders are not fully successful since they fail to recognise that the principle of moral arbitrariness is required to distinguish immigration restrictions as unjust where other (relational) inequalities may not be. Hence, the overall argument of this paper is that the recognition of the principle of moral arbitrariness is essential for the success of both the luck and relational egalitarian cases, and thus a proper recognition of the full normative force and implications of this principle entails the egalitarian case for open borders indeed succeeds. (shrink)