The 24-year-old played a key role for the Cottagers last season, playing in 36 Championship games as they reached the playoffs.
Fulham vice-chairman Tony Khan told the official club website: "It's a pleasure to welcome Tomas Kalas back to the Fulham Football Club.
"We appreciate Tomas so much after the way that he battled for us last season, demonstrating not only tremendous skill but also great heart and determination.
"We're very excited to have him here again as one of us as we prepare for our next battle, and what we hope will be a great season! COYW!"
Kalas, a Czech Republic international, is the 19th player to be loaned out by Chelsea already this summer.