The former Paris St Germain boss will take charge of the Blues for the first time less than 24 hours after his appointment as Frank Lampard's successor was confirmed.
The 47-year-old German has signed an initial 18-month contract to become the 15th manager of owner Roman Abramovich's tenure and his immediate task will be to reverse the fortunes of a team who have lost five of their last eight Premier League games and lie in 10th place in the table.
Tuchel has been itching to get back to work after his December sacking by PSG, and his agent Pini Zahavi has successfully sung his praises to Abramovich.
"It is never easy to change head coach in the middle of the season but we are very happy to secure one of Europe's best coaches in Thomas Tuchel," said Chelsea director Marina Granovskaia.
"There is still much to play for and much to achieve, this season and beyond."
Former Borussia Dortmund boss Tuchel must build a clear attacking strategy, find definitive roles for Werner and Havertz, tighten a tentative defence and also rebuild brittle collective confidence.
Chelsea's all-time record goalscorer, Lampard took the helm as manager in 2019 in a move to thrill Blues fans.
Lampard belied the club's transfer ban and previous paltry recruitment under Maurizio Sarri to seal a top-four finish in his first campaign.
The club's vast summer recruitment left Lampard with a bloated, imbalanced and callow squad, however, and the Blues' inconsistency ultimately led to his sacking on Monday.