A copper alloy seal matrix, which might have belonged to the ecclesiastic Petrus Philippi (d. 1341), has been found on the island of Biskops-Arnö in eastern central Sweden. Petrus Philippi was a Dominican, archbishop of Uppsala 1332–1341, and Arnö was the residence for him and his peers; a minor palace was built by his predecessor and completed by Petrus Philippi. Medieval ecclesiastical seal matrices areunusual finds, and a deliberately destroyed one like this one is even more rare. In this paper the seal matrix is presented and discussed.The authors present a hypothesis about the nature and date of the matrix and how and when it ended up in this particular spot. The matrix would not have been found if not forthe use of systematic metal detecting, the benefits of which are emphasized in this paper.