This study reveals the tactic of purification as a form of neo-colonial marginalisation present in contemporary development strategies on Indigenous lands. The research is based on my fieldwork study of exclusive tactics in a contemporary development conflict on Indigenous lands: the Arctic Railway project in Sápmi, in Northern Europe. The tactic of purification works through the selective use of opposites in excluding Indigeneity. On the one hand, ‘pure’ Indigeneity is an excuse for proponents of extractive development projects to exclude Indigenous knowledge and identities as ‘too Indigenous’ according to modern standards, denouncing them as ‘backward’, ‘only culture’, ‘not profitable’, or ‘without knowledge’. Yet, simultaneously, a resemblance to profitable livelihood practices, beyond culture, the use of several knowledge systems, and multi-ethnicity in communities, is deemed ‘too modern’, therefore ‘not pure enough’, thus invalidating Indigeneity. Building on classification systems introduced during colonialism, settler societies employ purification as a tactic to deny Indigenous peoples their right to decolonisation projects, and strengthen their control of Indigenous lands. The purification tactic thereby enables the expansion of the modern-colonial capitalist world order.