Forward Wes Thomas has signed for Birmingham City after rejecting a new deal with the Millers.
The 27-year-old joined the Millers from Bournemouth in January but only signed a contract until the end of the season.
Despite making 16 appearances for Rotherham – scoring six goals – as they won promotion, he rejected the offer of a new deal.
Thomas has signed a two-year deal with Birmingham, with whom he spent three months on loan during the 2012/13 season. The Blues had wanted to sign the striker at the conclusion of that loan, but could not afford the transfer fee.