The 21 year old joins the Millers on loan until the end of the season.
Koroma joined the Terriers last summer after turning pro at Leyton Orient.
Having made 9 appearances for Huddersfield this season, Terriers boss Danny Cowley is keen for him to get game time under his belt.
Speaking to the Huddersfield Examiner, Cowley said; "Josh is a really talented boy and we’ve been working hard with him on the training pitch to add different elements to his game."
"This is a good opportunity for him to make the next step in his development and to get regular minutes under his belt at a good standard in League 1."
"Rotherham are fighting for promotion, which will be a good experience for him, and we hope he will come back to us in the summer an even better player."