John Sinclair is a fictional character and the protagonist of a popular German horror detective series (of the dime novel or penny dreadful variety). The full title is Geisterjäger John Sinclair, (lit. Ghost Hunter John Sinclair), and the official English title is John Sinclair: Demon Hunter. The long-running series has been ongoing since 1973 and is released weekly since 1978. In the stories, Scotland Yard chief inspector John Sinclair battles all kinds of undead and demonic creatures, such as vampires, witches, ghouls and werewolves. While falling into the category of 'pulp fiction', the ongoing series is yet remarkable for the relatively wide range of its vocabulary and the inventiveness of its plots. The novels have been a commercial success in Germany, with over 250 million copies sold as of 2020. A recent milestone was the release of issue 2400 of the regular novel series in July 2024.

Most John Sinclair stories in the series have been written only by Helmut Rellergerd under the nom-de-plume of Jason Dark. Only a few of the earliest stories have been written by other authors. As of late, creation of new stories is divided among Rellergerd and several new authors, each of them writing separate full episodes; the new writers are attributed for their respective stories. For example, the Oculus duology (2017) was written by Wolfgang Hohlbein, while forensic biologist Mark Benecke was the author of the novel Brandmal. Rellergerd narrates the stories in first person from Sinclair's point of view whenever he is involved. At all other times a neutral narrator style is used. In some stories Sinclair does not appear and one of his friends takes the leading part.

The John Sinclair stories and their spin-offs Sinclair Academy and Dark Land are part of a larger shared universe of Bastei Lübbe novels and had multiple crossover events with several other German horror and fantasy series, such as Tony Ballard, Damona King and Der Hexer von Salem among others. The Professor Zamorra novels take place in an alternate universe within the fictional multiverse and also had numerous crossovers with John Sinclair since the 1976 story Der Fluch aus dem Dschungel.

Over the decades the Spanish painter Vicenç Badalona Ballestar has created numerous paintings and illustrations for the bestselling series. Other artists that contributed to artworks and cover designs for John Sinclair include Sebastià Boada, Tim White, Les Edwards, Luis Royo, Vicente Segrelles, Ron Walotsky, Alan M. Clark, Michael Whelan, Jim Warren etc.

In Germany, the stories have been adapted into multiple successful audio drama series and audiobooks, as well as a television film and a TV series. Additionally, the novels have also spawned a comic series, tabletop role-playing games and the 1998 video game John Sinclair: Evil Attacks. The novel series is being released in English as of April 2021 by J-Novel Club as part of their J-Novel Pulp imprint.