It is the 18-year-old's first professional deal.
The midfielder, from Edmonton, London, has been a key performer for the Blues Youth side.
He made 16 competitive appearances last season and hit the back of the net on five occasions. The previous season, he played 18 times for the Under-18s and scored once.
Thompson-Sommers came to Blues Academy via Spurs two years ago and earned a Scholarship.
A rangy midfielder who covers the pitch well and is very composed in possession, he gained experience with the Under-23s at the back end of last season as a squad member.