PSG said the signing was “fully in line with the club’s sporting strategy, which is heavily focused on youth and talent”.
Fernandez said his move was a “moment of huge pride” and PSG was “a massive club that I’ve followed since I was a child and where some great legends have made history”.
In a separate post on Instagram, he wrote: “After nearly four unforgettable seasons, today I have to make one of the most difficult decisions of my life: saying goodbye to the club that has been my home in recent years.”
Asked about Fernandez’s future earlier this month, Barca coach Hansi Flick said young players needed to “live for” the club and suggested the teenager would be making a mistake if he moved to France.
“We give them the opportunity to train with us, to grow, every day, with the best players in the world. We give them the opportunity, we give them the support, believe in them, the confidence,” he said.
“If you want to play for Barca, then [do it] 100% with your whole heart.
“This is what I want to say to everyone who is now with us, or in the future with us, this must be 100%.
“These colours, you have to live for them, this is what I want to see. All the others I don’t want, this is what I can say.
“Sometimes you do not agree with what they are doing, but they are 18, they are old enough to make the decisions, they have people around them.”
