Frederick III (or IV) (in Italian, Federico; 1 September 1341 - Messina 27 July 1377), called the Simple, was King of Sicily from 1355 to 1377. He was the second son of Peter II of Sicily and Elisabeth of Carinthia. He succeeded his brother Louis. The documents of his era call him the 'infante Frederick, ruler of the kingdom of Sicily', without any regnal number.