Isabel Diana Colegate (born 10 September 1931) is a British author and literary agent.
She was born in Lincolnshire, the daughter of Sir Arthur and Winifred Mary Colgate, and was educated at Runton Hill Schoolin Norfolk. She lived at Midford Castle near Bath, England, until 2007. In 1953 she married Michael Briggs, and the couple have two sons and a daughter.
Her novel The Shooting Party (1980) was adapted as an award-winning film of the same name, released in 1985 by Castle Hill Productions Inc. In 2010, it was adapted for radio by the BBC.