Grant has been linked with a move to Town throughout the January transfer window after attracting interest from the Yorkshire club and a number of other rival sides following a haul of 14 goals in 29 appearances for the Addicks.
After several bids, the Terriers finally forged a breakthrough and the 21-year-old has now moved to the club after successfully completing a medical.
Town head coach Siewert said: “As I do, this club believes in talented young players and Karlan certainly fits that description.
“He is a player I am delighted to have the chance to work with.
“Our recruitment team has put a lot of work into bringing Karlan here. Importantly, he is a natural goal-scorer; he has that knack of knowing which movement to make and finishing chances.
“He offers us something different to our current options in that he always tries to play on the shoulder of the final defender, looking to run in behind.
“He is an athletic, pacey player and will really fit our playing style.
“He is only 21 and he will improve here, for sure; he is an exciting English talent. He can play a big part in this club’s present and future.”