Havertz is the sixth player to join the Blues this summer, following the arrivals of Hakim Ziyech, Timo Werner, Ben Chilwell, Thiago Silva and Malang Sarr.
The 21-year-old Germany international was given permission to leave Germany’s training camp on Friday and caught a flight to the UK before heading to west London, where he put pen to paper on a five-year contract.
‘I am very happy and proud to be here,’ Havertz told Chelsea’s official website.
‘For me it is a dream come true to play in a big club like Chelsea and I can’t wait to meet all the players and the trainers.
‘Yeah, I am very happy to be here!’
Chelsea director Marina Granovskaia added: ‘Kai is one of the best players of his age in world football, so we are very happy that his future lies at Chelsea.
‘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.’
Widespread reports suggested Havertz would become the most expensive player in Chelsea’s history – with a £90m fee mooted at one stage – but Kepa Arrizabalaga keeps hold of the club record for the time being after his £72m switch from Athletic Bilbao in 2018.
Havertz had an outstanding individual campaign for Leverkusen last term, racking up 18 goals in 45 appearances for the German outfit, and his exceptional form after the Bundesliga’s return saw Chelsea step up their interest.
Frank Lampard has been determined to strengthen his attacking options ahead of the new season, despite already having a wealth of talent at his disposal.
Rennes’ Edouard Mendy has been identified as Chelsea’s priority target in this position but the club’s opening bid of £15m was rejected by the Ligue 1 side.
Chelsea are expected to return with an improved offer for the French shot-stopper.
It’s understood Rennes are demanding a fee of at least £31m for their prized possession.