The Germany international leaves Chelsea after three years with his name forever etched in the club's history.
Two moments from Havertz's Chelsea career stand out; he scored the only goal in the 2021 UEFA Champions League final against Manchester City and netted the winner when the Blues beat Palmeiras to lift the FIFA Club World Cup for the first time last year.
He also won the UEFA Super Cup during his time at Chelsea and scored 32 goals in 139 appearances.
Havertz is Arsenal's first summer signing, with more incomings expected at the Emirates Stadium.
'It's super exciting for me to be joining this amazing club and to be part of the Arsenal family. This club has such a big history, and I hope we can achieve lots of things,' Havertz, who will wear the No 29 shirt, said.
'The mentality in the Arsenal squad is very high and you can feel that. That was one of the reasons why it has been so hard to play against Arsenal recently.
'The aim is to win trophies and I'm going to give everything to do that for the supporters and everyone at the club. I'm now looking forward to meeting all the players and the staff when we come back for pre-season.'
Arsenal manager Mikel Arteta added: “Kai is a player of top quality. He has great versatility and is an intelligent player. He will bring a huge amount of extra strength to our midfield and variety to our play.
“We welcome Kai and his family to Arsenal Football Club and we are all excited to start working with him when we start pre-season training soon.”
Arsenal finished second in the Premier League last season, marking a return to the Champions League for the first time since the 2016/2017 campaign.
