The 22-year-old has signed a three-year deal with the Championship newcomers, with the club also set to bring in another wing-back option in the shape of Lincoln City's Cohen Bramall shortly.
Kioso - a Milton Keynes Dons academy product - started his career at non-league side Dunstable Town before moving to Hartlepool United.
Luton brought him to Kenilworth Road in January 2020 and he was initially signed as a development player with potential.
Kioso spent the 2020-21 season on loan at Bolton Wanderers and Northampton Town.
He returned to his academy club MK Dons on loan last August and impressed in 19 appearances for Liam Manning’s side last season, scoring four goals and providing three assists.
The player was recalled by Luton in January, with the Bedfordshire club suffering an injury crisis in defence and being left short on back-up for James Bree.
Dublin-born Kioso made 15 appearances in the second half of last season for the Hatters, who reached the Championship play-off semi-finals before losing out to Huddersfield Town.
Kioso is the Millers' second incoming arrival following the addition of Charlton Athletic frontman Conor Washington.
The club lost key duo Michael Smith and Michael Ihiekwe to Sheffield Wednesday on Thursday after the pair rejected fresh terms to join the League One club.