Sir William Martin Castell LVO FMedSci (born 10 April 1947) is a British businessman who was chairman of the Wellcome Trust, a director of General Electric and a former director of BP.
Castell was educated at St. Dunstan's College and Cass Business School, where he earned a BA.
He qualified as an accountant with Spicer and Pegler (1971–75) and then joined Wellcome plc as financial controller of Europe (1976–79), rising to MD Wellcome Biotech (1982–84) and commercial director (1984–89).
He joined the board of governors of the Wellcome Trust as chairman-designate on 1 January 2006, succeeding Sir Dominic Cadbury as chairman in May 2006.
He is a former trustee of the Natural History Museum, a board member of the University of Michigan's Institute of Life sciences, a board member of the National Bureau of Asian Research, a visiting fellow at Green College, Oxford University, and an Honorary Fellow of the Academy of Medical Sciences.