Sir David James George Hennessy, 3rd Baron Windlesham, Bt, CVO, PC (born 28 January 1932) is a Conservative politician in the United Kingdom, and currently holds visiting professorships at various universities.

Hennessy was educated at Ampleforth and Trinity College, Oxford, earning an MA in law. He joined the Westminster City Council in 1958, then held various positions before entering the House of Lords as the 3rd Baron Windlesham upon his father's death in 1968. He joined the government in 1970 as a Minister of State in the Home Office, and eventually became Lord Privy Seal and Leader of the House of Lords in 1973. He also began working with various companies, including The Observer, wrote a few books, and eventually went back to Oxford and earned a DLitt. In 1999, he was created Baron Hennesy (with one s) after the House of Lords Act 1999, so that he could continue sitting in the Lords. He is also a visiting professor at Princeton University and various other institutions.