"Nathan is a player I know a lot about and I think this is a really exciting signing for us," Rams boss Paul Jewell told his club's official website.
"At his best he can be the most lethal striker in the Championship.
"He has produced that form for me before and I believe I can get Nathan to reproduce that form for Derby and if he does he has the potential to be the best signing of the summer in this division.
"I tried to sign Jordan in the January transfer window but we ran out of time in the end.
"He is a great acquisition for us and someone who will have a big part to play in our promotion push next season."