Medieval manuscripts are among the oldest textual witnesses of spiritual life in the Low Countries. Therefore, from the outset of the library's establishement, the founders of the Ruusbroec Institute Library started collecting medieval manuscripts alongside early printed books. Starting an institutional medieval manuscript collection in the early twentieth century was not an easy task. Many manuscripts had already found a home in large institutional libraries like the Royal Library in Brussels or the university libraries in Ghent or Louvain. Nevertheless, up till now, the Ruusbroec Institute Library has managed to gather 34 medieval manuscripts and dozens of fragments up until now. Most of these manuscripts are in Dutch or Latin.

The Ruusbroec Institute Library has also over 400 post-medieval manuscripts, a richness that reflects  an often-overlooked culture of handwritten books.

Work on the online manuscript catalogue (in Dutch) is in progress and has started with the medieval manuscripts.