Magnus III of Mecklenburg (July 4, 1509 in Stargard – January 28, 1550 in Butzow) was a member of the House of Mecklenburg who was the first Lutheran bishop of the Prince-Bishopric of Schwerin. He was the son of Henry V, Duke of Mecklenburg and Ursula of Brandenburg. He was elected bishop in 1516, but due to his minority and lack of papal confirmation, he only began ruling as an administrator in 1532. He introduced the Reformation the next year. He married Elizabeth daughter of Frederick I of Denmark and Sophie of Pomerania. They had no children.