Smith and Ihiekwe looked primed to sign new contracts at the Millers but were tempted to remain in League One by big-money offers from Sheffield Wednesday at the 11th hour.
But they have made an addition with the capture of Kioso, who signs a three-year deal for an disclosed fee.
He becomes Rotherham’s second signing of the summer, along with Conor Washington, with more expected in the coming days.
Former Hull striker Tom Eaves looks set to join, having been seen at the AESSEAL New York Stadium this week.
Kioso, who is 22, is a right wing-back played 15 times for Luton last season as they missed out on promotion in the Championship play-offs while also spending time on loan at MK Dons, impressing boss Paul Warne.
He will join up with his new side for training on Friday.