[4] He then signed for Premier League champions Chelsea, on 7 July 2010, for a reported figure of around £5.2 million, although it was agreed that he would stay at Sigma Olomouc for the 2010–11 season.

During pre-season training, Kalas suffered fractured fibula, cutting his action with the first-team short as the rest of the team flew over to America for the Guinness International Champions Cup.

[14] After finally recovered from his injuries, Kalas made his competitive debut for the senior squad on 29 October 2013 as a substitute for Juan Mata, in a 2–0 win at Arsenal in the League Cup.

"[15][better source needed] On 18 March 2014, Kalas made his Champions League debut against Galatasaray, replacing Willian in stoppage time; Chelsea won 2–0.

[16][better source needed] On 27 April 2014, Kalas was given his debut in the Premier League for Chelsea, at Anfield against Liverpool, starting in place of the injured John Terry.

With the lack of playing time, Kalas was linked to a move to multiple English clubs, as well as his former team Vitesse in the January 2015 transfer window.

[24] Manager Aitor Karanka praised Kalas' aptitude to the right-back position, where Middlesbrough had been challenged following a long-term injury to Damià Abella in September 2014.

[27] On 14 April 2015, Kalas provided an assist to fellow Chelsea loanee Patrick Bamford as Middlesbrough beat Wolves 2–1 at home.

[29] Kalas played a big role in the Boro making it to the play-offs, although they were unable to beat Norwich City in the final to achieve promotion to the Premier League.

Even though he was playing a lot less, Middlesbrough manager Aitor Karanka stated that Kalas was still important in their chase for promote to the Premier League.

Kalas made an injury-time block to deny Matt Ritchie's goal-bound shot, as he helped Fulham hold out for the clean-sheet.

[42] Fulham qualified for the playoffs but lost 2–1 on aggregate in the semi-finals to Reading, with Kalas conceding the winning penalty by committing handball.

[46] On 23 August 2018, still in the Championship, Kalas joined Bristol City on a season long-loan along with fellow Chelsea loanee Jay Dasilva.

[47] On 1 July 2019, following his successful loan spell, Bristol City confirmed the permanent signing of Kalas for a club-record fee in the region of £8 million.

[50] Kalas made his debut for the U-17 on 5 September 2009 coming on as a 61st-minute substitute in a European Under-17 Championship Qualifying match against Belarus which the Czechs won 1–0.

[51] Kalas debuted for the Czech senior squad in a 3–0 friendly victory against Slovakia at his hometown's Andrův stadion on 14 November 2012, coming on as a substitute for David Limberský in the 74th minute.