Since the triple crises of food, fuel and finance of 2007/8, investments in agricultural growth corridors have taken centre-stage in government, donor and private sector initiatives. This article examines the politics of the multi-billion dollar development of the Southern Agricultural Growth Corridor of Tanzania (SAGCOT). The corridor’s proponents aim to create an environment in which agribusiness will operate alongside smallholders to improve food security and environmental sustainability, while reducing rural poverty. Based on three case studies, comprising one of a small-scale dairy company and two large-scale sugar companies, all operating with smallholders, this paper interrogates the political dynamics that shape the implementation of SAGCOT on the ground; in particular, the multiple contestations among bureaucrats, investors and smallholders over access to land and other resources, and contending visions for agricultural commercialisation. Despite the widespread support it received from government, donors and investors, the paper argues that SAGCOT’s grand modernist vision of the corridor, centred on the promotion of large-scale estates, has unravelled through contestations and negotiations on the ground.