Peter John Hennessy, Baron Hennessy of Nympsfield, is an English historian and academic specialising in the history of government.

He co-founded the Institute of Contemporary British History in 1986. From 1994 to 1997, he gave public lectures as professor of rhetoric at Gresham College, London. From 1992 to 2000, he was professor of contemporary history at Queen Mary and Westfield College, University of London. Since 2001, he has been Attlee professor of contemporary British history at Queen Mary.

Lord Hennessy has won many awards for his political writing.