The 17-year-old will stay at the Czech Republic side on a year-long loan deal.
Sigma Olomouc could earn around £5million from the sale of the Czech sensation.
Chelsea Academy boss Neil Bath said: “Tomas has signed, but will spend the first year on loan at his club, Sigma Olomouc, in the Czech Republic.
“He is now going into the Under 18s, and made his first-team debut last season. Very athletic and quick, he is a very good one-on-one defender and comfortable on the ball.
“We feel there is a lot of potential in the player, and we feel at the minute it is the right time to get him, but it would be good for him to stay there and get first-team experience, and we look forward to him joining us next year.
“Obviously we will stay in touch and he will come across here when appropriate. He is a right-sided centre-half who can play right-back.”
