Amelia Stewart, Viscountess Castlereagh

Amelia Anne Stewart, Marchioness of Londonderry (20 February 1772 – 12 February 1829), from 1794 until 1821 generally known as Emily Stewart, Lady Castlereagh /ˈkɑːsəlreɪ/, was the wife of the Georgian era Irish statesman Robert Stewart, Viscount Castlereagh, who from 1812 to 1822 was British Foreign Secretary and Leader of the House of Commons. Well-connected by birth to the aristocracy and wife of a prominent politician who was Britain's leading diplomat during the close of the Napoleonic Wars, Lady Castlereagh was an influential member of Regency London's high society.

Lady Castlereagh was a daughter of John Hobart, 2nd Earl of Buckinghamshire and his second wife, Caroline Conolly. The Earl of Buckinghamshire was an English courtier and politician who served as British Ambassador to Russia (1762–65) and Lord Lieutenant of Ireland (1776–80).

Her mother Caroline's father, William Conolly, was the nephew and heir of William Conolly, Speaker of the Irish House of Commons in the early 18th century and an extremely wealthy Irish landowner. Caroline's brother, Thomas Conolly, a noted Irish sportsman and Lady Castlereagh's uncle, was married to Louisa Lennox, daughter of Charles Lennox, 2nd Duke of Richmond, and one of the famous Lennox sisters. Caroline's mother, Anne Wentworth, was daughter of Thomas Wentworth, 1st Earl of Strafford.