The 23 year rejected the offer of a new contract with the Millers and after rejecting Millwall decided to join Bristol City, with the fee as yet to be agreed.
Ryan spoke today with bcfc.co.uk and explained the move.
"I met with the manager a few weeks ago and although there were other clubs interested, I was impressed with the ambitions of this club. City have a great following and some great names in the dressing room. I'm looking forward to working with the manager and the lads."
"It's a massive step up from League Two to the Championship and perhaps some believe it's too big, but plenty have done it before and I'm determined to make sure I'm successful. I'm a target-man and win my fair share of headers but I'm good at all aspects. I like to bring others into play and chip in with goals."
"It's hard to be leaving Rotherham United, having been there since the age of 16, but no one in their right mind would turn this chance down. I want to show everyone - the manager, coaching staff and fans - what I can do. I'll work hard and take my chance when it comes." he added.