He spent 18 months with the Terriers, helping them win promotion to the Premier League for the first time.
The 21-year-old is Derby's second signing of the winter as they continue their promotion push from the Championship.
Supporters can follow our new signing: @kaseypalmer45
Keep an eye out for all our #DeadlineDay content tomorrow, #dcfcfanspic.twitter.com/EBYMaKuSQY
— Derby County (@dcfcofficial) January 31, 2018