LaLiga side Barcelona added that the agreement does not include an option to make the transfer permanent.
Fati, a product of Barcelona's academy, enjoyed a breakout campaign for the Spanish heavyweights in the 2019-20 season, scoring eight goals in 33 appearances in all competitions.
Though his minutes were limited in the following two years due to injuries, he was a regular in Xavi Hernandez's side last season, making 36 league appearances as Barcelona won the LaLiga title. Fati has also played nine times for Spain.
He was a target for London club Tottenham Hotspur but his decision to move to the south coast is a major coup for Brighton, who have garnered a reputation for developing young talent.
"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," the club's technical director David Weir said.
"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."
Brighton finished sixth last season and will play in the Europa League in this campaign - their first-ever appearance in European competition.
Roberto de Zerbi's side, who are sixth in the Premier League standings, host Newcastle United on Saturday.
