The 20-year-old has exchanged the shore of Catalonia for the south coast in an eye-catching transfer.
"This is a great deal for all of us," De Zerbi said. "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 have made a habit of signing outstanding youngsters and making them a success in the Premier League, but to take an established international from a club like Barcelona breaks new ground for the club.
"We are delighted to bring Ansu to the club," said technical director David Weir. "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."