William Mackenzie, 5th Earl of Seaforth (died 1740), and 2nd titular Marquess of Seaforth (in the Jacobite Peerage), also known as Uilleam Dubh, or Black William, was a Scottish peer and head of Clan Mackenzie. Educated in France and brought up as a Roman Catholic, he was attainted for his part in the 1715 Jacobite Rising and also joined the 1719 Rising.