Gisilher, Gisiler or Giseler (died 1004) was the second Archbishop of Magdeburg, succeeding Saint Adalbert, from 981 until his death in 1004.
When he was elevated to the archdiocese in that latter year, however, he suppressed the Merseburg bishopric on the basis that its creation (967 or 968) had not received the written consent of the Diocese of Halberstadt.
In August, the margraves of Meissen, Lusatia, and the Nordmark joined with the troops of the Bishop of Halberstadt under the leadership of Gisilher and defeated the Slavs at Belkesheim, near Stendal.
In March 984, Gisilher hosted Henry II, Duke of Bavaria, in Magdeburg on Palm Sunday.
[5] On his death in 1004, there followed a brief conflict between King Henry II and the cathedral canons before Tagino was installed as archbishop.