The 24-year-old has scored 9 goals in 43 appearances for the Millers in his two-year tenure.
At the end of June, Millers boss Steve Evans announced the former Rotherham skipper was one of three players available for transfer from the south Yorkshire club.
Evans told www.Themillers.co.uk: "There is no doubt in my mind that Ryan has joined a good club and he will have a good season.
"As a manager you need to make sure that you get maximum benefit from the budget that the chairman puts in place, and this was also a clear factor as it is not a secret that I am still looking to add to the squad with a further three players."
