The Czech defender made two Premier League appearances for the Blues in 2013-14 but will continue his development at the newly-promoted Bundesliga club
The Czech, who has a solitary international cap to his name, spent two seasons on loan at Dutch outfit Vitesse Arnhem before returning to Stamford Bridge for the 2013-14 campaign.
After being handed his Chelsea debut as a late substitute in a 2-0 League Cup win at Arsenal in October, Kalas made his European bow in a last-16 home triumph over Galatasaray.
Kalas' first start for the club came in the huge 2-0 Premier League victory at Liverpool in April, and he was among the starting XI once again on the last day of the Premier League season at relegated Cardiff City.
The 21-year-old will now be hoping to pick up more first-team experience as Koln adjust to life back in the Bundesliga, after topping the second tier last term.
Kalas arrived at Chelsea from Sigma Olomouc in 2011.