The 23-year-old joined the Ligue 1 side on loan at the start of the 2025/26 season after successive injuries saw him slip out of the first team picture back in Catalonia.
The switch gave the Spain international a new lease of life as he netted 12 goals for the French side, at an impressive rate of one every 110 minutes, the best conversion rate in Ligue 1.
That form persuaded Monaco to push for a full transfer this summer and Fati has now signed a four-year contract until June 2030.
However, his departure from the Camp Nou has been far from smooth, amid a rumbling row over Barcelona's treatment of a player once heralded as being on the path to becoming a club legend.
After graduating through the famous La Masia system, Fati set records as La Liga's youngest-ever appearance maker and scorer - both of which have since been surpassed by Lamine Yamal - and his father has called out Hansi Flick's treatment of his son.
"He (Flick) never spoke to him, and nothing happened to make him reconsider his decision to leave.
"There were a few final details that needed to be agreed with Monaco, which is why the process took longer, but now it's finalised."
A fresh start in France could also revive Fati's international career - where he's not featured since 2023 - and only played a bit part role at the 2022 FIFA World Cup.