Middlesbrough sign Kalas on loan from Chelsea
Kalas spent the first half of the season on loan at Bundesliga outfit Cologne, but failed to make a single senior appearance.
The centre-back, who can also operate on the right side of the defence, will hope for regular first-team football with a Middlesbrough side pushing for a return to the Premier League.
Since joining from Sigma Olomouc, Kalas has made only two appearances for Chelsea, which came against Liverpool and Cardiff City last season.
But the 21-year-old does have a senior international cap, featuring in the Czech Republic's 3-0 friendly win over Slovakia in November 2012.
Kalas is available for selection for Middlesbrough's visit to Reading on Saturday.