Defender Matthew Pennington's goal clinched a 1-0 victory at Valley Parade that lifted the home side up to fourth. Rotherham remain 23rd after a fourth defeat in a row.
The Millers gave goalkeeper Ted Cann his league debut but he was beaten after 10 minutes as they were caught out by Max Power's quick free-kick.
Tyreik Wright was left in plenty of room to provide a cross from the left wing that was firmly headed home by Pennington.
Kayden Jackson was denied a second by a block from Jamal Baptiste before the Bradford striker went off injured.
Rotherham remained well in the game. Pennington deflected a Harry Gray effort past a post and Dru Yearwood's long-range shot took a touch over the bar.
Bradford went close just after the hour as Ethan Wheatley held the ball up for Jenson Metcalfe to unleash a first-time curling shot that flew past a post.
Metcalfe threatened again with a free-kick that clipped the top of the bar.
