Large-scale acquisitions of agricultural land in developing countries have been rapidly increasing in the last 10 years, contributing to a major agrarian transition from subsistence or small scale farming to large-scale com- mercial agriculture by agribusiness transnational corporations. Likely driven by recent food crises, new bioenergy policies, and financial speculations, this phenomenon has been often investigated from the economic develop- ment, human right, land tenure and food security perspectives, while its hydrologic implications have remained understudied. It has been suggested that a major driver of large-scale land acquisitions (LSLAs) is the quest for water resources that can be used (locally) to sustain agricultural production in the acquired land. The appropri- ation of water resources associated with LSLAs has often been termed ‘water grabbing’, though to date a formal definition of such a normative and inherently pejorative term is missing. The intrinsic assumption is that the ac- quisition of water undergoes the same dynamics of unbalanced power relationships that underlie many LSLAs. Here we invoke hydrological theories of “green” and “blue” water flows to stress the extent to which water ap- propriations are inherently coupled to land acquisitions and specifically focus on blue water. We then propose a formal definition of blue water grabbing based both on biophysical conditions (water scarcity) and ethical impli- cations (human right to food). Blue water grabs are appropriations of irrigation (i.e., blue) water in regions affect- ed by undernourishment and where agricultural production is constrained by blue water availability. We use this framework to provide a global assessment of the likelihood that LSLAs entail blue water grabbing.