Elizabeth Wynne Fremantle (born 19 April 1778 at Falkingham, now Folkingham, Lincolnshire, died 2 November 1857 in or near Nice, France) was the main author of the extensive Wynne Diaries and wife of the Royal Navy officer Thomas Fremantle (1765–1819), a close associate of Nelson.
Known in the family as Betsey, she was born Elizabeth Wynne, the second daughter of Richard Wynne (1744–1799) and his wife Camille (born de Royer, died 1799), who were Roman Catholics. Wynne was a fast liver, later a friend of Casanova. He got into financial difficulties in 1786, sold his Lincolnshire estate, and took his family abroad. Elizabeth married Fremantle in 1797, after he had rescued her and her family from Leghorn (Livorno) during the 1796 French invasion of Italy and taken them to safety in Corsica. The marriage took place in Naples on 12 January 1797, at the house of the British envoy, Sir William and Lady Hamilton, who took care of the arrangements.
Thomas Fremantle bought the manor of Swanbourne, Buckinghamshire, for his family in 1798 for 900 guineas. Elizabeth lived there for the rest of her life. The Fremantles' children included Thomas (1798–1890), a Tory politician later created 1st Baron Cottesloe, a daughter Emma (born 13 June 1799),Charles (1800–1869), a Royal Navy officer after whom the city of Fremantle in Western Australia is named, William Robert (c. 1809–1895), who became Anglican dean of Ripon in Yorkshire, and Stephen Grenville Fremantle (1810–1860), captain in command of HMS Juno from 1853 to 1858.
Privately, Elizabeth lost sympathy with Lady Hamilton and Nelson when their conspicuous affair became known: "I had a letter from my husband today.... Lady Nelson is suing for a separate maintenance. I have no patience with her husband, at his age and such a cripple to play the fool with Lady Hamilton." Fremantle, in command of the Ganges, distinguished himself at the Battle of Copenhagen in 1801 under Nelson's command. He was also prominent at Trafalgar in 1805. Elizabeth bore her husband's absences at sea with difficulty, especially as her family grew. They kept up an intimate correspondence, which is spliced into the 1952 edition of the diaries. They received a good deal of hospitality from the family of Lord Buckingham, who lived nearby at Stowe. Buckingham as a figure close to the government was a help to Fremantle in his naval career.