Frederick Spencer, 4th Earl Spencer KG, CB, PC (14 April 1798 – 27 December 1857), styled The Honourable Frederick Spencer until 1845, was a British naval commander, courtier, and Whig politician.
He initially served in the Royal Navy and fought in the Napoleonic Wars and the Greek War of Independence, eventually rising to the rank of Vice-Admiral.
He succeeded his elder brother as Earl Spencer in 1845 and held political office as Lord Chamberlain of the Household between 1846 and 1848 and as Lord Steward of the Household between 1854 and 1857.
In 1849 he was made a Knight of the Garter.
Through his second son, Charles, Lord Spencer was the great-great-grandfather of Diana, Princess of Wales, the three-times-great-grandfather of future British Monarch Prince William, Duke of Cambridge, and the four-times-great-grandfather of William's son, Prince George.