In this paper a sentence from a chronicle of Jan Plastwich , a canon of the Warmia Chapter, was anlysed: [...] et isti dixerunt, se audivisse a progenitoribus suis et etiam ab illis, quorum progenitores habitaverunt in terra Galindiae, quod maior rex Galindiae habitaverit circa magnum lacum nomine Nabentine, qui est ad medium miliare ab illa parte Leiczen versus Litwanos, et vocabatur rex ille Ysegups. The analysis has confirmed that indeed there was a Galindian chieftain called Ysegups. The Warmian chronicler has derived this information from a brief mention made for the court proceedings between the Teutonic Order and the Bishopric of Warmia. The case concerned the affiliation of former Galindian lands, and took place during the reign of bishop Jan of Miśnia, probably around 1353–1354. Old Prussian witnesses during their testimony about the course of Galindian borders have told about Ysegups. He had his seat on the lake Niegocin, half a mile from a place called Leiczen, in the direction of Lithuania. Leiczen probably coincides with the present day Piękna Góra. Here the first Teutonic castle was built and after the relocation of the keep to the site of present day Giżycko, the former location was called Alt Lötzen, Stary Lec or Stari Zamek. So the seat of Ysegups is to be found within the borders of the present day town of Giżycko, maybe around the castle, where already during the course of archaeological excavations, some wooden remnants were found. The name Ysegups is probably Old Prussian and it means “an expatriate”. As all of the information about Ysegups has proven to be true, there is no reason to doubt the validity of the claim that he was indeed the high king of Galindia (maior rex Galindiae). The only period when there were high kings of whole Old Prussian tribes was the time of the Great Uprising of 1260–1274, so it is probable that Ysegups could have been such a high king of Galindia from the time of Teutonic conquest up to the depopulation of the area in the years 1268–1272.