The versatile defender, who can operate in central defence and at right-back, spent the second half of last season at Middlesbrough, playing 17 times for the Sky Bet Championship club.
Earlier in the 2014/15 campaign, Kalas went on loan to Bundesliga side Cologne after spending two seasons on loan with Dutch side Vitesse Arnhem.
The 22-year-old has already played for Chelsea’s first team, making his debut in the 2-0 Capital One Cup win over Arsenal in October 2013.
Kalas was given special permission by the Blues to join the Teessiders on their pre-season training trip to Marbella, where he put pen to paper on Friday morning.
Now Kalas, who missed last season’s play-off games because his loan deal had come to an end, is hoping to avoid the end-of-season lottery.
He told Boro’s official website, www.mfc.co.uk: “Obviously I wanted to be involved. Those were something special at the end because you were playing for promotion and I didn’t have a chance to help the team.
“It was a bit disappointing I couldn’t play, but this is a new challenge and hopefully we can get promoted without the play-offs.”
Kalas revealed he did not have to think twice after being offered the chance to return to the north-east.
He said: “I’d already decided at the end of last season that if Chelsea would like to get me on loan again, this would be my first choice, so it was easy when I heard I could stay.
“It’s been easy to adapt because I know the lads. I know Marbella because we came here last year, so it’s been easy.
“I think we’re still building the team. It’s a mix of everything. Everyone is still working on it and I think we will be stronger than before.”
Kalas, who joined Chelsea for around £5m from hometown club Sigma Olomuc in 2010, was part of the Czech Republic side which competed at the recent European U21 Championships on home soil.