Bundesliga strugglers Borussia Dortmund have agreed to sign Slovenia international Kevin Kampl in January.
Cardiff have agreed to sell Norwegian midfielder Mats Daehli to Freiburg in January.
The 27-year-old Poland international has made over 140 appearances for Forest since arriving in the summer of 2009.
Terriers boss Mark Robins said: "We have made it clear throughout the close season that we would stay patient during the transfer window and today (Tuesday) that patience has paid dividends with the arrival of Radoslaw.
"He is a top-class Championship player, especially in the number 10 role, and we're delighted to bring him to the club."