The Martinus Library is the third largest library in Mainz with around 300,000 volumes and one of the largest theological libraries in Germany. It is open to anyone interested.

Due to its unique historical holdings, the institution is a renowned ecclesiastical and academic library among scholars and students in Mainz and the Rhine-Main area, as well as in Germany and abroad. Its main areas of focus are:

• theology,
• church history,
• History,
• German literature
• as well as the latest literature on theology and current religious, ethical, and social issues.

The Martinus Library is the oldest library in Mainz (since 1662) that has been in continuous operation to this day. Its holdings include 200 current newspapers, 903 incunabula, 300 manuscripts, and the "Schlossersche Bibliothek" with important original editions, especially of German literature from the 16th to 19th centuries.

Modern working methods are facilitated by the online catalog and bibliographic apparatus, including CD-ROM research and the Internet, as well as the reading room with 20 workstations.