Although the transparency and sustainability of governments are currently of great interest to researchers, few studies have specifically addressed these issues. Nevertheless, previous research has found sustainability transparency as a key issue in government–citizen relations, especially for local governments (LGs). The aim of this article is to identify factors that promote online transparency regarding the sustainability of public policies, by means of an empirical study of 62 LGs in the United Kingdom, Ireland, and Southern Europe. Our results show that the prevailing administrative tradition may influence the degree of transparency of LGs, with population, socioeconomic, and financial factors being relevant.