King of the Slavs (Latin: rex Sclavorum, Sclavorum rex) was a title denoting some Slavic rulers, as well as Germanic rulers that conquered Slavs, in the Middle Ages in European sources, such as Papal correspondence.
Papal use is bolded.
SlavicSamo, ruler of Slavs (623–658); in the Frankish AnnalsDrogoviz, ruler of the Veleti (789); in Annales Mettenses priores in c. 805Trpimir I, ruler of Croatia (845–864); erroneously by Gottschalk in the 840sSvatopluk I of Moravia, ruler of Moravia (870–894); by Pope Stephen V in 885Michael, ruler of Zahumlje (913–926); erroneously in the Annales BarensesMihailo Vojislavljević, ruler of Duklja/Montenegro (1050–1081); by Pope Gregory VII in 1077Vukan, ruler of Rascia/Serbia (1089–1105); by William of Tyre between 1170 and 1184Stefan Dragutin, ruler of Serbia (1276–1282) and Syrmia (1282–1316); by Pope Nicholas IV in 1288Non-SlavicCanute Lavard, Danish prince (1120–1131); by Abbott Wilhelm after 1129