Thomas Sydserf [Sydserff] (b. 1581; d. 1663) was a 17th century Scottish prelate. The eldest son of an Edinburgh merchant, Sydserf graduated from the University of Edinburgh in 1602 before travelling to continental Europe to study at the University of Heidelberg. After returning to Scotland, he entered the ministry, beginning at St Giles' parish, Edinburgh in 1611. 15 years later, in 1626, he was translated to Trinity College church, Edinburgh, before being admitted Dean of Edinburgh on February 19, 1634.
However, in the same year, and on the recommendation of William Laud, Archbishop of Canterbury, he ascended to episcopal rank, receiving consecration as Bishop of Brechin on July 29. In the following year, on August 30, 1635, he was translated as Bishop of Galloway. Sydserf was very much a royalist, pro-Episcopacy, and inclined to be highly sympathetic towards Arminianism. These views brought him much conflict in Scotland, and a Bishop of Galloway he exercised his episcopal powers against his ideological opponents. He supported the introduction in 1637 of an English-style Book of Common Prayer, and for this he was attacked on several occasions by mobs in Falkirk, Dalkeith and Edinburgh. He was finally deposed by the rebellious General Assembly of the Scottish church on December 13, 1638.
Sydserf thereafter went to England, briefly becoming a follower of King Charles I, before moving continental Europe. He returned to Scotland, and after the Restoration and reimposition of Episcopacy in Scotland, was reinstated as a Bishop, though on this occasion becoming Bishop of Orkney. He was the only pre-1638 bishop to be reinstated as a bishop in Scotland after the Restoration. Sydserf died in Edinburgh on September 29, 1663. He had been married since 1624, when he took as his wife Rachel Byers, daughter of an Edinburgh magistrate. He was responsible for remodelling the nave of Whithorn Priory in line with the new styles or worship he tried to promote.