William Douglas, 4th Duke of Queensberry

William Douglas, 4th Duke of Queensberry KT ( born December 16, 1725 † December 23, 1810 ), called " Old Q", was a Scottish nobleman.

William Douglas was the son of William Douglas, 2nd Earl of March, a grandson of William Douglas, 1st Duke of Queensberry. His mother was Anne Hamilton, daughter and heiress of John Hamilton, 1st Earl of ruglen. William succeeded his father in 1731 as Earl of March, 1748 his mother as Earl of ruglen.

William was Lord of the Bedchamber of King George III. since in 1760. During the illness of the king in 1788, he advocated the reign of the Prince of Wales, after recovery of the king he was therefore dismissed in 1789.

In 1778 he inherited his father's cousin, and was the Duke of Queensberry. The Duke was known for its zealous promotion of horse racing as well as for its casual lifestyle, although he left numerous illegitimate children and over one million pounds; however, had never married and left no legitimate heirs. The Duchy of Queensberry fell to Henry Scott, 3rd Duke of Buccleuch, a grandson of Jane Douglas, daughter of James Douglas, 2nd Duke of Queensberry. His mother's title lapsed with his death.