William Douglas, 4th Duke of Queensberry

William Douglas, 4th Duke of Queensberry, (16 December 172423 December 1810) was a British peer and courtier. He was popularly known as Old Q (because that letter was painted on his carriage's door) and was reputed as a high-stakes gambler. He was appointed a Knight of the Thistle in 1761 and was one of the 16 Scottish representative peers for an elected term or possibly more from 1761, and was Vice Admiral of Scotland from 1767 to 1776. However, due to behavior during the king's unusual, long-lasting, mental health latter-life illness he was deprived of his office as Gentleman of the Bedchamber in 1789, and for a while took refuge abroad. Later, he was Lord Lieutenant of Dumfries from 1794 until 1810.

He succeeded his father in the Earldom of March in 1731 and his mother in the Earldom of Ruglen in 1748. He succeeded his cousin Charles as Duke of Queensberry in 1778, and was created Lord Douglas, Baron Douglas, of Amesbury in the County of Wiltshire in the Peerage of Great Britain on 8 August 1786.

In 1799 he was estimated the eighth-wealthiest man (or small family unit) in Britain, owning £1M . He was one of ten known British millionaires in 1799. In 1798, she became the wife of the 3rd Marquess of Hertford; Queensberry left much of his wealth to Maria, He was interred at St James's Church, Piccadilly on 31 December 1810.

On his death, the dukedom and Drumlanrig Castle passed to his second cousin once removed, the third Duke of Buccleuch. The Marquessate of Queensberry passed to his fourth cousin once removed (and also third once removed) Sir Charles Douglas, 5th Bt, whose descendant is the current titleholder. His second cousin twice removed Francis Douglas, 8th Earl of Wemyss became Earl of Wemyss and March. The Earldom of Ruglen became extinct.

Horseracing He was a racehorse owner and event attendee. His jockeys' racing silks were deep red with a black cap. He had some society repute as a high-stakes gambler.