The research reported here aimed to ascertain the degree of transparency exhibited by Spanish public university libraries based on their active public disclosure practice. Transparency was measured by applying the TransPa_BA tool to the transparency-related information published by the country’s 50 public university libraries on their websites. The tool addresses 21 indicators grouped under eight areas, used in this study to measure university libraries’ public disclosure performance. The data collected were scored pursuant to the provisions of Spanish Act 19/2013 of 9 December on Transparency, Access to Public Information and Good Governance as adapted to university libraries, as well as in terms of other factors scantly developed in or omitted from the Act. The indicators and their respective parameters (content, form, accessibility, reusability, dating and updating or validity) constitute guidelines enabling libraries to enhance transparency and accountability by reporting their activities and practice in key areas to society in general and their stakeholders in particular. The objective is to help such institutions operate more transparently, for the information afforded by the indicators is deemed relevant to their activity while also monitoring their performance. The findings show that university libraries, which have become more transparent over time, are more transparent than other information units (national public libraries and historic and university archives). Some were nonetheless found to have room for improvement. On the grounds of their scores, these libraries can be classified under three headings: transparent, translucent or opaque. The parameters where good practices were most frequently identified included service usage rules and regulations, user charters and the annual report.