The 29-year-old attack-minded midfielder joins Matt Mills as a new face in the Reds squad, signing a four-year deal with the club.
He made 33 appearances for the Rams last season and in total netted 29 league goals in 125 games after moving permanently from Sheffield United in the summer of 2011.
Ward netted Derby's equaliser against Forest when the two teams met in January 2013 and also scored a controversial goal in September 2011, playing on when Chris Cohen was down injured.
Having been on international duty with Northern Ireland into June, he will link up with Dougie Freedman's squad on Monday and get to know his new colleagues during a pre-season camp in Sweden.