The agreement includes an option to buy, and comes alongside news that Fati, 22, has signed a new contract extension with Barcea through 2028, signaling the club's continued long-term belief in the player's potential.
Monaco's interest - and the promise of Champions League football - proved decisive in Fati's decision, according to sources familiar with the deal.
"Ansu was drawn to the project Monaco presented," a source told ESPN. "He wants to play consistently, stay fit, and get back to competing at the highest level - especially in Europe."
Once heralded as one of world football's brightest young talents, Fati has seen his career stall due to recurring injuries and limited minutes. Under Hansi Flick at Barcelona last season, he featured in just 11 matches across all competitions, starting only three and logging 297 minutes in total.
The lack of game time led to the decision to seek another loan spell - this time in France - in hopes of reigniting the form that saw him become a first-team star at age 16.
Fati burst onto the scene in 2019 with a string of record-breaking performances. He remains Barcelona's youngest ever Champions League goalscorer and the youngest in the competition's history, scoring at just 17 years and 40 days - a record he still holds ahead of current rising star Lamine Yamal.
However, a series of injuries between 2020 and 2022 disrupted his rhythm and development. He was loaned to Brighton and Hove Albion in the 2023-24 season, where he made 27 appearances and scored four goals in the Premier League.
Despite showing flashes of his talent in England, his role at Barcelona remained limited upon returning.
Fati's contract renewal through 2028 suggests that Barcelona still believe in the forward's upside, but acknowledge the need for him to gain consistent match experience elsewhere.
In total, he has scored 29 goals in 123 appearances for the Catalan club since his debut. On the international stage, he has 10 caps and two goals for Spain, but hasn't featured for the national team since 2023.
The move to Monaco represents a crucial chapter for Fati - a chance to reset, compete for silverware, and make a case for both club and country as he enters a pivotal stage in his career.