Grant, who was also linked with Championship side Brentford, has scored 14 league goals for Charlton in the third division this season.
New Huddersfield manager Jan Siewert is seeking reinforcements in his bid to lead the basement club to Premier League survival this season.
"Our recruitment team has put a lot of work into bringing Karlan here," Siewert said in a statement.
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"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's an athletic, pacey player and will really fit our playing style."
In the German's first match in charge, Huddersfield suffered a 1-0 league defeat by Everton on Tuesday, leaving the club 11 points from safety.
Huddersfield confirmed Grant will be available for Saturday's league trip to Chelsea.
(Reporting by Hardik Vyas in Bengaluru; Editing by Amlan Chakraborty)
