Kalas, 22, made 17 league appearances for the Championship side last season after joining on loan in January.
The Czech, who can play at right-back or centre-half, has played four times for the Premier League title holders.
"I'd already decided at the end of last season if Chelsea would like to get me out on loan this would be my first choice," he told the club website.
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Middlesbrough have signed Chelsea defender Tomas Kalas on a season-long loan deal.