When not properly implemented, foreign direct investment (FDI) can produce many adverse environmental, labour, and human rights impacts that risk curbing the host state’s development efforts. Facing this issue, the research introduces a new concept, foreign investor diligence, to contribute to closing the governance and accountability gaps associated with FDI. The concept is defined as an autonomous set of socially expected standards of conduct adopted by a “model” foreign investor in its relationship with stakeholders affected by its FDI activities in the host state to ensure its alignment with sustainable development objectives. Looking specifically at the extractive sector, the research outlines the content and application of foreign investor diligence under two main components. First, it sets out the cartography of foreign investor diligence, by mapping the social norms enshrining standards of expected behaviours relating to FDI. This is done by focusing on international instruments enshrining direct obligations for foreign investors, international soft law instruments targeting corporate actors, and industry-specific standards and business practices. Second, the research tries to identify ways to make foreign investor diligence “actionable” against foreign investors, by looking at specific avenues for actionability, such as investor-State dispute settlement, foreign direct liability litigation, and non-judicial grievance mechanisms, and setting out the legal concept that can offer “entry points” to consider foreign investor diligence within the identified mechanisms.