The deal is worth around £71 million, according to BBC Sport.
Havertz joins attacker Hakim Ziyech, striker Timo Werner, left-back Ben Chilwell, and center-back Thiago Silva as Chelsea's latest big-name signing, and reportedly brings the club's total spending this summer to around £200 million.
The 21-year-old signed a five-year contract with the Blues.
"Kai is one of the best players of his age in world football, so we are very happy that his future lies at Chelsea," director Marina Granovskaia said in a statement. "He has proven pedigree in one of the best leagues in Europe, he plays for the German national side and he is an exciting, dynamic talent. We are delighted to be able to add his versatility and quality to the squad before the season begins."
Havertz lit up the Bundesliga, scoring 15 goals and assisting seven others in 2020 alone. He became the youngest player in league history to reach 100 club appearances and the first to score 35 career goals before the age of 21.
He even wore the captain's armband in the absence of oft-injured skipper Lars Bender, making him the youngest player to lead a Bundesliga club since Lukas Podolski in 2005.
Havertz started his journey at Leverkusen as an 11-year-old and made his senior debut at age 17 in October 2016. Call-ups to the German national team soon followed.
Bayern Munich and Liverpool were linked with Havertz, but Chelsea eventually solidified their interest with a bid. Leverkusen reportedly rejected an opening offer of £58.6 million plus add-ons before agreeing to a deal.
Known as a versatile midfielder who can play on the right and up the middle, Havertz is likely to form a fruitful partnership with Werner at Stamford Bridge.
Manager Frank Lampard could play Werner on the left wing or at the striker position, with Havertz set to start as the No. 10 in a 4-2-3-1 formation. Ziyech, the Moroccan international signed from Ajax, is expected to wreak havoc on the right wing.
Chelsea made every attempt to retool their squad this summer, making up for lost time following a transfer ban. The club won its appeal against the embargo in December but waited until the end of the 2019-20 season to spend big.