The 27-year-old moves to The City Ground from Fulham having helped The Cottagers to promotion to the Premier League last season.
Christie started out at Coventry City and made over 100 appearances for the Sky Blues before departing for Derby County. He spent three years with The Rams ahead of signing for Middlesbrough and then going on to help Fulham win promotion via the play-offs in 2017-18.
After a season in the top flight, Christie helped Fulham return at the first attempt last season and the Republic of Ireland international now joins up with the Nottingham Forest squad for the 2020-21 campaign.