The England youth international signed initially on a month's loan, and has had it extended to the end of the season.
Bell-Baggie started his first game in the Millers last game at Aldershot and has already impressed Millers supporters.
The 17yrold winger spoke to the official site saying: "I am now looking forward to the rest of the season, getting more game time and helping Rotherham to promotion.
"I want to try and grab this opportunity with both hands, train hard every single day and try and impress the gaffer."