Arthur Augustus William Harry Ponsonby, 1st Baron Ponsonby of Shulbrede was a British politician, writer, and social activist. He was the third son of Sir Henry Ponsonby, Private Secretary to Queen Victoria, and the great-grandson of Frederick Ponsonby, 3rd Earl of Bessborough. Frederick Edward Grey Ponsonby, 1st Baron Sysonby, was his elder brother.
Lord Ponsonby is possibly best remembered for the statement: "When war is declared, truth is the first casualty", which he made in his book Falsehood in Wartime: Propaganda Lies of the First World War. A similar line previously had been spoken in 1917 by US Republican Senator Hiram Johnson.