Undocumented migrant women in Europe – many potential refugees – face cumulative forms of discrimination and heightened vulnerability, including the risk of suffering violence. This article explores the implications of viewing their situation through the lens of public international law, particularly international human rights law, as an indispensable companion to European human rights law. It argues for the incorporation of the UN normative and interpretative framework on the human rights of undocumented migrants into European legal and judicial analysis. The UN legal architecture constitutes an essential component to better understand the transnational phenomenon of migration in an interrelated world with combined and overlapping legal regimes. These instruments crafted within the realm of public international law may also play a key orienting role for the ECtHR to adequately uphold the rights of (female) undocumented migrants and for States to attend more justly to the dire conditions of many undocumented migrant women and girls in Europe.