The 24-year-old has played more than 50 times for the Blues, including in their Champions League final victory over Bayern Munich in 2012.
Villa boss Paul Lambert said: "Ryan brings a lot of big-game experience, not only in the Barclays Premier League but even a Champions League final which Chelsea won, so he'll be really good for us.
"The way he plays the game and as a person, too, he'll fit in here great and it will be good for him also to be playing games. He has real enthusiasm for the game and he's looking forward to playing here.
"We're grateful to Chelsea for allowing him to come here on loan and we know he'll be really good to work with."
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