State-led mega-development projects in ecologically sensitive regions increasingly depend on religious institutional endorsement to secure public legitimacy. This article examines how Indonesia’s two largest Islamic mass organisations, Nahdlatul Ulama (NU) and Muhammadiyah, discursively mediate the legitimisation of Ibu Kota Nusantara (IKN), the government’s controversial new capital city project in East Kalimantan, despite documented ecological risks including deforestation, biodiversity loss, and indigenous community displacement. The study adopted an eco-critical discourse analysis (eco-CDA) framework, combining the three-dimensional model of Norman Fairclough with the eco-linguistic techniques of Arran Stibbe. It analysed public discourse from national, regional, and organisational actors in the period 2022–2024. The results show that both organisations are discursive brokers that diffuse environmental opposition through different but complementary strategies of legitimisation. NU sacralises growth, using religious imagery to frame IKN as a spiritual duty. This leads to pseudo-sustainability, when green language acts as a moral sanction rather than an ecological critique. Muhammadiyah applies a procedural-pragmatic strategy that contextualises the encounter in terms of institutional expansion. This is demonstrated by passive eco-silence, the systematic discursive absence of ecological discourse. These findings help explain when religious civil society organisations are vehicles of hegemonic developmental normalisation and when they are champions of ecological accountability.