 Foster signed for League Two side Rotherham United on 20 June 2011. He signed a one-year contract with the club, with an option of a second year. Foster made his debut for the club in a 3 – 0 home victory against Gillingham on 27 August 2011, coming on as an 84th-minute substitute with Rotherham already holding a three-goal advantage. After making no further first-team appearances the following month, Foster was made available for loan by manager Andy Scott. Scott stated Foster lacked both desire and fitness since signing for the club — "If you want to be match fit, you get yourself match fit. You can work in training, you can stay behind, you have days off, you can get yourself fit. Luke has not been in the side and that tells you everything you need to know". However, no loan move materialised, and Foster made his second appearance for Rotherham in a 3 – 3 away draw at Morecambe on 25 October 2011. He appeared as a 27th-minute substitute in the club's following match four days later, a 1 – 1 draw against Northampton Town, and subsequently made his first start in a 2 – 0 home victory over Aldershot Town on 5 November — Rotherham's first win in 10 matches. In December 2011, Rotherham manager Andy Scott revealed that Foster would miss the rest of the 2011 – 12 season as a result of a cruciate ligament damage sustained in a match against Hereford United on 10 December 2011. He was released by Rotherham in May 2012, having made six appearances for the club.
