Thomas Sydserf [Sydserff] (1581 – 1663) was a 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 Kirk, Edinburgh, before being admitted Dean of Edinburgh on 19 February 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 29 July. In the following year, on 30 August 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. Some went further and accused him of being a Roman Catholic: he was alleged to wear a crucifix. He was finally deposed by the General Assembly of the Scottish church on 13 December 1638.