An undisclosed fee has been paid for the Wales under-21 international whose exit comes after a raft of defensive signings made by Reds boss Sean O'Driscoll over the summer.
"We're pleased to bring Kieron to Derby County and he fits the bill in terms of the type of player we are looking to bring into the squad," Rams boss Nigel Clough told his club's official website.
"He's young, he has potential and we've been impressed with what we've seen of him, whether it be at Forest or while he was out on loan at both Mansfield and Notts County.
"He certainly increases our options in defence and midfield and we're confident he'll settle in just fine."
Some smart work in the transfer market allied to a solid start to the season means the impending closure of the transfer window is unlikely to see the panic buying for Forest that it did last year.
Instead the focus is set to be on those players who do not fit into O'Driscoll's plans.
One of those players, Ishmael Miller, is being linked with both Blackpool and Middlesbrough while a loan move to Coventry is seen as possible for David McGoldrick.