The highly-rated 20-year-old forward is moving to Roberto De Zerbi's side in search of regular playing time.
Fati, who inherited Lionel Messi's number 10 jersey at Barcelona, has struggled with injuries in recent seasons and most of his 36 La Liga appearances in the last campaign came from the bench.
The Spain international will hope to regain his best form at the AMEX Stadium this season before returning to Barcelona next summer.
An official statement from the Spanish champions read: “FC Barcelona and Brighton Hove Albion Football Club have reached an agreement for the loan of the player Ansu Fati until 30 June 2024. There is no option for a permanent transfer.”
Brighton boss De Zerbi said: "This is a great deal for all of us. I am sure Ansu will help us to reach a new target and we can help him get back to the level he deserves to be."
Brighton technical director David Weir added: "We are delighted to bring Ansu to the club. He is one of the most highly-rated young players in the world and arrives with an impressive amount of experience both domestically and in European competition, despite his age.
"This season is the biggest in the club's history and we're really happy that a player of Ansu's quality is going to be part of it. We will give him the opportunity to embed himself in the squad and to get used to working with Roberto, but naturally we're excited to see him in a Brighton shirt."
Brighton have been drawn in the same group with Ajax Amsterdam, Olympique Marseille and AEK Athens in the Europa League.
