Societal interest in all aspects of sustainability has increased. Therefore, pig farmers need to be aware of their strengths and weaknesses in all dimensions of sustainability: economy, environment, social wellbeing, and animal health and welfare. Our aim was to describe and critically discuss the development of a sustainability assessment tool for pig farms and to evaluate its suitability by applying it to 63 European pig farms (13 breeding, 27 breeding-to-finishing, and 23 finishing farms). The multi-criteria assessment tool was developed in several steps (the selection and scaling of indicators and their aggregation and weighting) in order to summarise the indicators into subtheme and theme scores. The indicators contributing the most to the subtheme/theme scores were identified and discussed in order to evaluate the procedure of the development. For example, some indicators, such as Ecological compensation area, Fairness of prices, and Tail docking, for which farms were scored low, were also identified as “real world problems” in other studies. For other sustainability aspects with low performance, the threshold might have been set too ambitiously, e.g., for Number of sows per annual working unit. Furthermore, to analyse the suitability of the tool, we assessed the best and worst median theme scores (good and poor performances) for each dimension, as well as the variability of the performances of the farms within the themes. Some themes were found to be moderate, such as Pig comfort, Biodiversity, or Resilience, whereas others were found to be good, e.g., Water and the Human–animal relationship, as well as several themes of the social wellbeing dimension. Overall, the sustainability tool provides a comprehensive assessment of the sustainability of pig production. Furthermore, this publication contributes to both the theory (development of a robust sustainability tool) and the practice (provision of a tool to assess and benchmark the sustainability on farms). As a next step, a sensitivity analysis should be performed, and the tool should be applied for further development.