ABSTRACT Drawing motivation from the importance of water for life on Earth in current times and the past, this article investigates the role of accounting in favouring the sustainability of water supply in medieval times by discovering and integrating its financial, social, and environmental dimensions. This period was characterised by demographic and economic growth, which led to increasing water needs and interventions for water supply. We analyse a historical case study, whose conventional accounting is reinterpreted according to the ‘accounting for sustainability’ framework. The case of reference is the city of Siena (Italy) in medieval times with its peculiar water supply system, the ‘Bottini Aqueduct’, undertaken primarily during the thirteenth century. First, this article delves into the pivotal role of conventional accounting in unveiling and monitoring the financial, social, and environmental dimensions of water supply interventions; second, by proposing the metaphor of the ‘tree of life’, it emphasises the role of accounting in favouring the integration of these dimensions by supporting the incorporation of environmental resources – such as water – in the life of society and economy, as sap flows from the roots to the branches in a living tree.