George Spencer, 4th Duke of Marlborough (January 26, 1739) - (January 29, 1817) was a British nobleman. Born the son of Charles Spencer, 3rd Duke of Marlborough, he was known as Marquess of Blandford until succeeding his father in 1758. He was educated at Eton, and in 1755 entered the Coldstream Guards as an Ensign, becoming a Captain with the 20th Regiment of Foot the following year.

After inheriting the dukedom, Marlborough took his seat in the House of Lords in 1760, becoming Lord Lieutenant of Oxfordshire that same year. The following year, he bore the sceptre with the cross at the coronation of George III. In 1762, he was made Lord Chamberlain as well as a Privy Counsellor, and after a year succeeded this appointment as Lord Privy Seal.

The Duke was made a Knight of the Garter in 1768, and was elected to the Royal Society in 1786. He married Lady Caroline Russell, daughter of John Russell, 4th Duke of Bedford, in 1762, by whom he had seven children. He died at Blenheim Palace aged 78, and was buried there.

|- style="text-align: center;" | width="30%" |Preceded by:
The Duke of Bedford | width="40%" style="text-align: center;" |Lord Privy Seal
1763–1765 | width="30%" |Succeeded by:
The Duke of Newcastle