The 22 year old signed a 3 year deal with the Millers after Luton accepted a bid from the Millers..
Kioso made 19 appearances for the Hatters last season where they made the Championship play offs as well as a spell on loan at MK Dons, where he featured against the Millers.
Kioso will join his new team for pre-season tomorrow. (friday)
Speaking to the Advertiser, boss Paul Warne said; "He's athletic, quick and can defend really well one v one. He's also good in the air.
"We'll need to get more out of him in the final third but he just adds a bit more for us down that side of the pitch because currently we only have Wes and Chieo there. He is in to compete with those two in those sorts of positions."