Frederick William Robert Stewart, 4th Marquess of Londonderry KP PC (7 July 1805–25 November 1872), styled Viscount Castlereagh from 1822 to 1854, was a British nobleman and Tory politician.

Frederick's mother was the fourth and youngest daughter of John Bligh, 3rd Earl of Darnley.

He was the only child of his parents but had younger half-siblings: Following the death of his mother and during his father's absence on military and diplomatic duties, Frederick was largely raised by his uncle and aunt, Lord and Lady Castlereagh.

[5] He served under the Duke of Wellington as a Lord of the Admiralty from 1828 to 1830 and under Sir Robert Peel as Vice-Chamberlain of the Household from December 1834[6] to April 1835.

By 1852, he "had fallen out with his father, the 3rd Marquess of Londonderry over their views on the land question [and] was obliged to retire because of these differences".

His wife, the dowager Marchioness of Londonderry died on 2 September 1884, aged 70, and was buried with him in the double grave in the priory.

This had the effect that the fortunes of the Stewart and the Vane side of the Londonderry family were reunited in a single hand.