The 21-year-old, who spent 16 months at the Etihad Stadium, scored 16 goals in 43 appearances for City in all competitions and he departs a Premier League and Carabao Cup winner, having helped the Club lift both trophies in his debut 2020-21 campaign.
This season, his progress was curtailed by a metatarsal injury suffered whilst on international duty, which has kept him out since October.
Director of Football Txiki Begiristain said: “Ferran should be proud of what he has achieved here at Manchester City.
“Last season was his first time in a new country, but he adapted well. He always gave 100%, worked hard for the team and scored goals which helped us win trophies.
“Injury unfortunately prevented him from playing more games this season, but Ferran is a player we have all enjoyed working with and we wish him all the best at Barcelona and for the rest of his career.”
Torres arrived at the Etihad in August 2020 and made his debut as a substitute as City beat Wolves 3-1 on the opening day of last season.
A full debut in a Carabao Cup victory over Bournemouth followed and he scored his first goal for the Club in the following round, with a clinical curling effort in the 3-0 eclipse of Burnley.
The youngster went on to score a further time seven times before Christmas, including four goals in five Champions League group stage games as Guardiola’s men qualified for the knockout stage with an unbeaten record.
He ended his debut campaign with 13 goals in 36 appearances, with his standout performance coming in a 4-3 win at Newcastle United, when he scored a hat-trick, which included a spectacular finish that won the Nissan Goal of the Season.
Torres made a positive start to the 2021-22 campaign, scoring three times in seven games, including a brace in a 5-0 rout of Arsenal, before injury struck whilst representing Spain in the Nations League Finals.
It means his final appearance for the Club came three months ago, when he scored the fourth goal in a 6-1 win over Wycombe Wanderers in September.
Everyone at Manchester City sends their best wishes to Ferran ahead of his move to Barcelona.