William of Villehardouin (; Kalamata, 1211 – 1 May 1278) was the fourth prince of Achaea in Frankish Greece from 1246 to 1278. The younger son of Prince Geoffrey I, he held the Barony of Kalamata in fief during the reign of his elder brother Geoffrey II. William started to rule Achaea as regent for his brother during Geoffrey's military campaigns against the Greeks of Nicaea who were the principal enemies of his overlord, the Latin Emperor of Constantinople Baldwin II. William succeeded his childless brother in the summer of 1246. Conflicts between Nicaea and Epirus enabled him to complete the conquest of the Morea in about three years. He captured Monemvasia and built three new fortresses, forcing two previously autonomous tribes, the Tzakones and Melingoi into submission. He participated in the unsuccessful Egyptian crusade of Louis IX of France who rewarded him with the right to issue currency in the style of French royal coins.