The novel will be a prequel to the game, which, for the first time, sees the action taking place in Afghanistan instead of during World War II. The game is inspired by real events in the country, which was invaded by the US in 2001 in response to the September 11 attacks.
In previous versions of 'Medal of Honor', players assumed the role of an Allied soldier. In the new, R-rated game, the action is seen through the eyes of a solder in an elite squad called Tier 1.
Ryan's novel will be given away with pre-ordered copies of the game, as well as being sold in bookshops. The game launches on 15 October for PlayStation3, Xbox and PC.
Ryan's first novel was 'The One That Got Away', a best-seller about the notorious Bravo Two Zero mission during in the first Gulf War, from which Ryan was the only soldier to escape.
He said: "Bringing together the disciplines of literature and videogames has been a new but compelling experience for me as a writer, resulting in what is hopefully an engaging read for fans of videogames and novels alike."
Yesterday retailers were urged not to sell the game by defence secretary Liam Fox, because it lets players take the part of the Taliban and kill British troops.
The 'Medal of Honor' series launched in 1999, with numerous new versions appearing on various formats since then. Steven Spielberg, director of 'Saving Private Ryan', is one of the series' creators.