Kalas a key figure in Fulham's promotion charge last season
Kalas spent last season on loan with the Cottagers and made 40 appearances in all competitions as the club reached the play-off semi-finals, where they were defeated over two legs by Reading.
The Czech centre-back was an integral part of Fulham's promotion push last time around, and is looking to go one step further this season as the club look to return to the Premier League for the first time since they were relegated in 2014.
Fulham's Vice Chairman and Director of Football Operations, Tony Khan, told the club's official website that it is a "pleasure to welcome Tomas Kalas back to Fulham Football Club."
Khan continued by saying the club were grateful for the way the 24-year-old "battled for us last season", praising his "heart and determination" during their promotion push.
Khan concluded by saying that the Cottagers are "very excited to have him here again" ahead of their first fixture of the 2017/18 Championship season, which takes place on August 5 at home to Norwich City.
Czech defender one of many Chelsea loanees
Since joining Premier League champions Chelsea from hometown club Sigma Olomouc in 2010, Kalas like many Blues youngsters has found himself on loan more often than not.
Two seasons with Chelsea's feeder club in the Netherlands, Vitesse Arnhem, saw him named as Czech Republic's Young Player of the Year before he enjoyed sporadic appearances in England as he made his Premier League debut for the Blues against Liverpool during the 2013/14 season.
Further loan spells with Bundesliga side 1. FC Koln and Championship outfit Middlesbrough followed in the 2014/15 season as Kalas helped the Boro reach the play-off final, before he would spend the following season on Teesside once more where he won promotion to the top-flight.
The 24-year-old then spent last season on loan with Fulham as manager Slaviša Jokanović mounted a promotion charge, and has returned to South-West London in the hopes of securing a return to the top-flight at the second time of asking.