Many studies examine the correlation between the use of resources such as water, energy and land, and the production of food. These nexus studies focus predominantly on large-scale systems, often considering the social dimensions only in terms of access to resources and participation in the decision-making process, rather than individual attitudes and behaviors with respect to resource use. Such a concept of the nexus is relevant to urban agriculture (UA), but it requires customization to the particular characteristics of growing food in cities, which is practiced mainly at a small scale and produces not only food but also considerable social, economic, and environmental co-benefits. To this end, this paper proposes a new conceptual basis for a UA Nexus, together with an assessment methodology that explicitly includes social dimensions in addition to food, energy, and water. The conceptual basis introduces People, together with Food, Energy, and Water, as a fundamental factor of the UA Nexus. On this basis, a methodology is developed measuring not only resource efficiency and food production but also motivations and health benefits. It comprises a combination of methods such as diaries of everyday UA practices, a database of UA activities, life cycle assessment (LCA), and material flow analysis to connect investigations developed at a garden scale to the city scale. A case study shows an application of the methodology.