The Denmark international decided against signing a new contract at Chelsea, and he officially became a free agent when his Stamford Bridge deal expired at the end of June.
Christensen has now been confirmed as Barcelona's latest acquisition, with the centre-back following Franck Kessie to Camp Nou, with the Ivory Coast international moving on a free transfer from AC Milan.
Speaking at his official unveiling, the 26-year-old said that he is "very proud" to have joined Barcelona and has thanked the senior figures at the club for making the move possible.
"I'm very proud," Mundo Deportivo quotes Christensen as saying. "It's a dream that I've had for a long time. I'm very grateful to the president, the vice president, Mateu [Alemany] and Jordi [Cruyff]."
Meanwhile, Barcelona president Joan Laporta also welcomed the defender to the club during the unveiling, calling it "a very special day".
"Welcome to Andreas Christensen," Mundo Deportivo quotes Laporta as saying. "It's a very special day. Thank you for your efforts and for the simple things you have in your life. Christensen wrote when he was eight years old that he wanted to be a Barcelona player.
"Here we want players who dream of playing for Barcelona and we're grateful for that. I want to congratulate everyone who's worked on this signing. Thanks for the work you've done. The desire he had to come to Barcelona was positive. He comes from a great club and we're very satisfied."
Christensen was an important player for Chelsea during the 2021-22 campaign, making 34 appearances in all competitions, contributing two goals and one assist in the process.
Meanwhile, the Dane made 160 appearances for the Blues in all competitions, becoming a regular at Stamford Bridge after returning from a loan spell at Borussia Monchengladbach.
Ronald Araujo, Gerard Pique and Eric Garcia will again be options for Barcelona in the middle of the defence next season, but there will be outgoings to make room for Christensen's wages.
Oscar Mingueza, Samuel Umtiti and Clement Lenglet are all expected to leave Camp Nou this summer, with the latter believed to be on the verge of joining Tottenham Hotspur.
Christensen could make his debut for the Catalan giants in their pre-season opener against Olot on July 13, while the club will also take on Inter Miami, Real Madrid, Juventus and New York Red Bulls ahead of the new campaign.
Barcelona, who finished second in La Liga last season, will begin their 2022-23 campaign against Rayo Vallecano on August 13 before travelling to Real Sociedad one week later.
The Catalan outfit are still working to sign Robert Lewandowski from Bayern Munich, with the club's president Joan Laporta revealing that a bid has been submitted to the German club.