William Cavendish, painted by Sir Thomas Lawrence, 1811.

William George Spencer Cavendish, 6th Duke of Devonshire, KG, PC (21 May 1790 – 18 January 1858), was known as the "Bachelor Duke".

Born in Paris, he became Duke of Devonshire at the age of 21 on the death of his father in 1811. His mother, Georgiana Cavendish, Duchess of Devonshire, had died in 1806. Along with the title he inherited eight stately homes and 200,000 acres (809 km² or 80,900 ha) of land. He went on to improve his houses and gardens (including the rebuilding of the village of Edensor) and travelled extensively.

He was Lord Chamberlain to King William IV and a close friend of Czar Nicholas I of Russia. Among his friends were also Antonio Canova, Charles Dickens, and his head gardener, Sir Joseph Paxton.

The 6th Duke had two sisters, both of whom married well. One of his nieces Lady Blanche Howard married his heir presumptive, the 2nd Earl of Burlington (grandson of Lord George Cavendish, created 1st Earl of Burlington, younger brother of the 5th Duke), and thus their children were also great-grandchildren of the 5th Duke. Unfortunately, Lady Burlington died young. Her widower did not remarry, even when he succeeded the 6th Duke in 1858.

Much of the 6th Duke's private correspondence, including letters to his mistresses (one of whom he installed nearby), was destroyed by his Victorian relatives. The Duke is said to have wanted to marry The Princess Mary, a daughter of George III, but she preferred to marry her Royal cousin the 2nd Duke of Gloucester (son of her uncle by his wife, who was born an illegitimate commoner).

The Duke had no children so when he died, at the age of 67, the dukedom passed to his cousin William Cavendish, 2nd Earl of Burlington.

There is a biography by James Lees-Milne: The Bachelor Duke: Life of William Spencer Cavendish, 6th Duke of Devonshire, 1790-1858 (1991). John Murray. ISBN 0-7195-4920-5