The Chelsea defender will get another season of first team football in the North-East.
The 22-year-old, who has won one cap for the Czech Republic, featured 17 times for Boro last season, and will have a chance to add to that total again when the new campaign begins in August, with Sky saying the paperwork is being prepared to finalise the deal.
Kalas arrived at Stamford Bridge in 2011, after coming through at Sigma Olomouc, and spent his first couple of seasons at Vitesse Arnhem.
The centre-back starred in his first few appearances for the Blues, helping Chelsea go to title favourites Liverpool late in the 2013-14 season and secure a 2-0 win.
He was sent to Cologne on loan at the start of last season but failed to make an appearance for the Bundesliga side, returning to the UK and taking part in Middlesbrough's ultimately failed attempt to secure promotion to the Premier League.
But the Czech international impressed in his few months at the Riverside Stadium, and with Aitor Karanka determined to go one better next season and secure automatic promotion, Kalas will surely be a vital part of that challenge.