Demon Internet was a British Internet service provider, initially an independent business, later operating as a brand of Vodafone. It was one of the UK's earliest ISPs, offering dial-up Internet access services from 1 June 1992. According to the Daily Telegraph, it "sparked a revolution by becoming the first to provide genuinely affordable access to the internet in the UK".

In 1997 Demon was bought by Scottish Telecom, a wholly owned subsidiary of the private utility company Scottish Power. Scottish Telecom rebranded as Thus plc in October 1999 and floated on the London Stock Exchange. Thus plc fully demerged from Scottish Power in 2002. Thus became part of Cable & Wireless plc, and then part of Cable & Wireless Worldwide following a split of its parent. The company was purchased as part of the acquisition of Cable & Wireless Worldwide by Vodafone Group on 27 July 2012. Demon then operated as a brand of Vodafone.

From 1996 to 2006 Demon operated a subsidiary ISP business in the Netherlands. It was sold to KPN in June 2006 and its operations transferred to their XS4ALL subsidiary.

In January 2019, Vodafone announced its intention to close Demon and migrate its 15,000 remaining customers to more modern services.