The 22-year-old only featured twice for the Swans after his move from Arsenal last summer, spending more than three months on the sidelines with a thigh injury.
His chances of regular playing time this season were not helped by the recent signing of Espanyol centre-back Jordi Amat and Swansea have opted to send him out on loan to gain more first-team experience.
Bartley was regularly loaned out when he was with Arsenal, having two spells with Sheffield United and Rangers.
"It's great news. As soon as my agent mentioned the move then I was all for it because this is a massive club with a massive fan base. I know the manager well and he's a really good young manager who matches the club's ambition," Bartley told the club's official website.
The Blues have also been linked with a move for another Premier League defender, Fulham's Dan Burn.