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.
Slavic
Samo, ruler of Slavs (623–658); in the Frankish Annals
Drogoviz, ruler of the Veleti (789); in Annales Mettenses priores in c. 805
Trpimir I, ruler of Croatia (845–864); erroneously by Gottschalk in the 840s
Svatopluk I of Moravia, ruler of Moravia (870–894); by Pope Stephen V in 885
Michael, ruler of Zahumlje (913–926); erroneously in the Annales Barenses
Mihailo Vojislavljević, ruler of Duklja/Montenegro (1050–1081); by Pope Gregory VII in 1077
Vukan, ruler of Rascia/Serbia (1089–1105); by William of Tyre between 1170 and 1184
Stefan Dragutin, ruler of Serbia (1276–1282) and Syrmia (1282–1316); by Pope Nicholas IV in 1288
Non-Slavic
Canute Lavard, Danish prince (1120–1131); by Abbott Wilhelm after 1129
