Does practice demonstrate the legitimacy of international human rights law? This article explores this question via a case study of the women’s rights movement in Iran. Current human rights sceptics question the system’s legitimacy because of a lack of universality and an excessive top-down approach. However, the Islamic Republic of Iran has a remarkable community of grassroots activists. The bottom-up women’s rights initiative of the One Million Signatures Campaign utilised human rights discourse in combination with local, indigenous values in pursuit of gender equality. The article argues—via the case study of this movement—that there is practical evidence to support a theory of human rights universalism, as positive human rights law empowers the existing subjectivity of individuals. The universal legitimacy of international human rights law does not primarily come from a global network dictating common values, but from members of civil society mobilising their status as rights holders.