The 24-year-old defender, who has had loan spells with five other clubs since arriving at Stamford Bridge in 2006, moves to Paul Lambert's side until the end of the season.
"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,” Villa boss Paul Lambert told the club’s official website. "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."
Bertrand is buoyed by the move to Villa Park and is relishing the prospect of regular playing time under Lambert.
"It's a fantastic opportunity for me to come here. Aston Villa is a fantastic club with a massive fanbase and a great history. To get this chance to come here and play some games will be a really good step for me," said the defender. "Whenever I've played for Chelsea here at Villa Park the fans have been fantastic and have always created a great atmosphere. The pitch, too, has always been great, including when we played in the Community Shield here against Manchester City, and I'm looking forward to playing on it more regularly until the end of the season.
"I've had to adjust many times in my career having played under several managers, so I know I can settle in here quickly and I'm looking forward to being able to contribute to the team. Knowing some of the lads here, too, such as Fabian Delph, Marc Albrighton and a few others will really help, too, and I couldn't be more looking forward to playing my part here."