The survival of medieval manuscripts in their original bindings remains a rare occurrence. Taking advantage of the diversity of bindings in Cistercian libraries such as Clairvaux and its daughter abbeys during the twelfth and thirteenth centuries, this study focuses on the biocodicological analysis of medieval manuscript bindings, with particular emphasis on the use of sealskins. Using innovative methods such as electrostatic zooarchaeology by mass spectrometry (eZooMS) and ancient DNA (aDNA) analysis, this research identifies the animal species and origin of the leather used in these bindings as predominantly pinniped (seal) species. In particular, the collagen-based eZooMS technique facilitated the classification of seven chemises into the pinniped clade, although species identification remained elusive, except in one additional case where a bearded seal () was definitively identified. aDNA analysis was instrumental in verifying the origin of the sealskins, with four samples identified as harbour seals and a single sample as a harp seal and sourced to (contemporary) populations in Scandinavia, Scotland and Iceland or Greenland. This geographical inference supports the notion of a robust medieval trade network that went well beyond local sourcing, linking the Cistercians to wider economic circuits that included fur trade with the Norse. The study, therefore, highlights the use of an unexpected skin (seal) from an unexpected source (the northwestern Atlantic). The widespread use of sealskins in Cistercian libraries such as Clairvaux and its daughter abbeys during the twelfth and thirteenth centuries hints at broader trade networks that brought, for example, walrus ivory from the far north into continental Europe. This integration of the biological sciences into the study of historical manuscripts not only provides a clearer picture of the material culture of medieval Europe, but also illustrates the extensive trade networks that Cistercian monasteries were part of, challenging previous assumptions about local resource use in manuscript production.