Despite their imposing material presence, the values and harms stemming from the construction of infrastructural megaprojects remain speculative affairs in Istanbul, Turkey. This article distinguishes two modes of speculation pertaining to megaprojects that present different ethical and political possibilities, namely de-materialising and re-materialising speculation. Contributing to debates about material politics informed by Noortje Marres (Material Participation; Palgrave Macmillan, 2015) and William Connolly (The Fragility of Things; Duke University Press, 2013), our framework of de- and re-materialising speculation avoids isolating material and planetary agency to instead consider how their uncertainties and potentials play a role at the intersection with politico-economic life. Accordingly, we analyse the coalescence of ecological and democratic crises. In Turkey, populism’s anthropocentric construction of “the will of the people” exacerbates ecological collapse. But also, ecological collapse inspires a search for a politics and ethics that acknowledge human–nonhuman ecological entanglements. The contestation between de-and re-materialising speculation underscores possibilities and limitations of ecological politics in contexts of populism and post-truth.