Ansu, who turned 17 in October, only signed his first professional deal with the club in July but those terms were agreed with the assumption he would be playing for the under-19s and B team.
He has since broken into the first team, featuring 11 times this season, and forcing Barca to update his contract and improve his salary for the second time in six months. His buy-out clause will automatically rise to €400m when he's officially promoted to the first team.
"For me, La Masia [Barca's academy] is the best school in the world," Ansu said. "They helped me from the first day I arrived and I am extremely grateful to everyone who works there.
"They are all spectacular. From the first day, I have learned things from everyone. And they have also been there to support me through difficult times."
"Working every day with the best is a dream, and also a reward. From now onwards, what I have to do is keep working, even harder than ever, to keep getting chances and carry on enjoying myself. The rest will come after."
While he is training on a day-to-day basis with Ernesto Valverde's side, he is not yet been registered as a first-team player and did not appear in last week's official squad photograph.
Ansu signed his new contract at the club's offices on Wednesday. President Josep Maria Bartomeu, sporting director Eric Abidal and director of youth football Patrick Kluivert were all present as he put pen to paper.
The club negotiated the deal with Lionel Messi's brother, Rodrigo, who works as Ansu's agent and is an adviser to the youngster's father, Bori.
Ansu made his debut in Barcelona's second La Liga game of the season, a 5-2 win over Real Betis. A week later, he became the club's youngster ever scorer in La Liga -- aged 16 years, 298 days -- when he netted in the 2-2 draw against Osasuna.
In his first start, against Valencia at Camp Nou in September, he became the youngster player to ever score and assist a goal in the same game in La Liga.
The Royal Spanish Football Federation subsequently fast-tracked the process to him to obtain citizenship and he has since made his debut for Spain U21s. He was also eligible for Guinea-Bissau and Portugal.