Thorfinn Sigurdsson (1009?–c. 1065), also known as Thorfinn the Mighty, was an 11th-century Earl of Orkney. He was the youngest of five sons of Earl Sigurd Hlodvirsson and the only one resulting from Sigurd's marriage to a daughter of Malcolm II of Scotland. He ruled alone as earl for about a third of the time that he held the title and jointly with one or more of his brothers or with his nephew Rögnvald Brusason for the remainder. Thorfinn married Ingibiorg Finnsdottir, daughter of Finn Arnesson, Jarl of Halland.