The 22-year-old, who has yet to feature for the Tigers this season, moves to the New York Stadium as a short-term replacement for the injured Lionel Ainsworth.
Millers boss Steve Evans said: "[Devitt] is predominantly a right winger, but he can be very effective on the left, we have seen him in both areas of the pitch.
"He is somebody we have known for a couple of years. He is not getting a game in the Championship, but is hungry to play and has great ability.
"He played in Hull's reserves on Tuesday and was their best player."
Devitt could make his debut in Saturday's FA Cup tie with Stevenage.