Marcus Williamson (born 1965) is a British writer, journalist and campaigner, living in the UK.
In 2009 the AIM-listed spyware company Phorm created a website Stopphoulplay.com to attack Williamson and fellow campaigner Alexander Hanff. The company accused Williamson of "...waging a "serial letter writing" campaign to Phorm's potential customers and partners in attempt to discredit the company and Mr Ertugrul."  The site was soon taken down and later described as a "PR disaster".
He is the editor of the consumer information website CEOemail.com.
He advocates contacting company chief executives (CEOs) as a means of resolving customer service issues and of conducting campaigns. The site was featured in the LifeHacker article "CEOEmail.com Helps You Take Your Issue to the Top When Other Avenues Fail".
- The True Celtic Language and the Stone Circle of Rennes les Bains. 2008. ISBN 978-1257639526. Translation of an 1886 work by Henri Boudet.
- Claude Cahun at School in England. Self-published, 2011
- Path. Atelier St Louis Production, 2011. With Jonathan Moss. ISBN 978-0-956833-40-2
- Neate, Rupert (28 Apr 2009). "Phorm chief labels critics 'serial agitators'". The Telegraph.
- Neate, Rupert (May 1, 2009). "Phorm's Stopphoulplay site is a PR disaster". The Telegraph.
- Insley, Jill (14 September 2010). "Nationwide chief executive shuts down email address". The Guardian.