Lieutenant-General George FitzRoy, Duke of Northumberland, KG, PC (28 December 1665 – 28 June 1716) was the third and youngest illegitimate son of King Charles II of England ('Charles the Black') by Barbara Villiers, Countess of Castlemaine (also known as Barbara Villiers, Duchess of Cleveland); he was the fifth of Charles's eight illegitimate sons.

On 1 October 1674, he was created Earl of Northumberland, Baron of Pontefract (Yorkshire) and Viscount Falmouth (Cornwall).

Catherine was the widow of Thomas Lucy of Charlecote Park, a captain in the Royal Horse Guards.

Soon after the marriage, Northumberland and his brother, Henry FitzRoy, 1st Duke of Grafton, allegedly attempted to privately convey her abroad to an English convent in Ghent, Belgium.

The Duke lived at Frogmore House at Windsor in Berkshire, but died suddenly aged 50 at Epsom on 28 June 1716.