Gwendolen Florence Mary Guinness, Countess of Iveagh (née Onslow; 22 July 1881 – 16 February 1966) was an Anglo-Irish aristocrat and Conservative politician. She was, by marriage, a member of the Guinness brewing dynasty.

She was the daughter of William Hillier Onslow, 4th Earl of Onslow (1853–1911), and Florence Coulston Onslow, née Gardner (1853–1934).

She was married to the Conservative Member of Parliament (MP) for Southend, Rupert Guinness, 2nd Earl of Iveagh. In 1927, he ceased to be an MP when he succeeded to his father's earldom. The Countess of Iveagh, as Gwendolen Guinness was now known, won the Southend by-election on 19 November 1927 to replace her husband as MP.^[1] She received 54.6% of the vote at that election^[2] and increased it at the 1931 general election. She served until her retirement at the 1935 general election.

When she retired in 1935, she was succeeded as MP by Henry "Chips" Channon, the husband of her eldest daughter, Honor Guinness. Another son-in-law, Alan Lennox-Boyd, was an MP (for Mid Bedfordshire 1931–60, and thus became, with Gwendolen, the first mother- and son-in-law pair of MPs.^[2]

In 1956 she presented her Surrey childhood home, Clandon Park House, to the National Trust.

• 1881 – 1903: The Lady Gwendolen Florence Mary Onslow
• 1903 – 1919: The Lady Gwendolyn Guinness
• 1919 – 1927: Viscountess Elveden
• 1927 – 1966: The Right Honourable The Countess of Iveagh