William Rees may refer to:
- William Rees (MP), in 1390 MP for Norfolk
- William Hurst Rees (1917–2004), British surveyor and member of the Lands Tribunal
- William James Rees (1913–1967), British hydroid and cephalopod researcher
- William E. Rees (born 1943), originator of "ecological footprint" concept
- William Rees (cinematographer) (1904–1961), American cinematographer
- William Rees (Archdeacon of St Asaph) (1905–?), later Welsh Anglican priest
- William Rees (Gwilym Hiraethog) (1802–1883), writer
- William Rees (priest and writer) (died 1936), earlier Welsh Anglican priest
- William Rees (rugby), rugby union and rugby league footballer of the 1920s and '30s
- William Rees (veterinarian), Chief Veterinary Officer of the United Kingdom, 1980–1988
- William Lee Rees (1836–1912), English-born New Zealand cricketer, politician and lawyer
- Billy Rees (1924–1996), association footballer of the 1940s and '50s
- William Gilbert Rees (1827–1898), explorer, surveyor, and early settler in Central Otago, New Zealand
- William Rees (Canadian physican) (1800-1874), a public health official and advocate for immigrant health in Toronto
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