Freeman has put pen to paper on a two-year deal with the Rams, with the option of a further year, after the two clubs agreed an undisclosed fee last week.
The 20-year-old, who can play at full-back or wide midfield, will feature for Derby's Under-21 team away to Leeds on Wednesday afternoon.
"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," said Rams boss Nigel Clough. "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."
Freeman came through the youth ranks at Forest. He had loan spells at Mansfield and Notts County last season, spending the second half of the campaign across the banks of the River Trent.