Messi, 34, will wear the number 30 as revealed in a video by the club, the number he had when he began his professional career at Barcelona between 2004-2006.
“I am excited to begin a new chapter of my career at Paris Saint-Germain,” said Messi in a statement on the club’s website.
Messi is joining other stars at PSG such as France forward Kylian Mbappe, Italy goalkeeper Gianluigi Donnarumma, Spain center-back Sergio Ramos and Brazil’s Neymar to complete a super team.
It comes after Messi arrived in the French capital to a hero’s welcome and undergoing a medical.
The 34-year-old, who was a free agent after his shock departure from boyhood club Barcelona last week, landed at Le Bourget airport to the north of Paris on Tuesday afternoon.
There he was greeted by hundreds of PSG supporters who had gathered hoping to catch a glimpse of their new signing.
Those fans also gathered outside the club’s Parc des Princes stadium and outside a plush hotel in the city where Messi along with wife Antonella and their three children are expected to be staying.
Messi waved to crowds while sporting a Paris t-shirt on his arrival at the airport before being taken for a medical at a hospital just outside the city, the final step before his signing could be made official by the Qatar-backed club.
Messi was taken by a police motorcade to the hospital for his medical assessment.
Messi’s father Jorge, who is also his agent, earlier confirmed the inevitable as he arrived at Barcelona’s El Prat airport to board a flight to France.
Asked by reporters if his son would sign for the French club, he replied: “Yes”.
PSG’s move for Messi had been an open secret for days, although reports in Spain on Monday night talked of a “final offer” by Barcelona to try to keep the player.
Instead, the six-time Ballon d’Or winner will join a PSG attack already featuring Neymar and Kylian Mbappe.
PSG see Messi as the missing piece in their jigsaw as they chase the Champions League, the trophy they want more than anything else.
“Back together,” Neymar, who played with Messi at Barcelona before moving to PSG in 2017, posted on Twitter.
Barcelona are still coming to terms with the departure of their greatest ever player, who said goodbye to his club of two decades at a tearful news conference on Sunday.
Despite offering to cut his salary by half to seal a new five-year contract with Barcelona, the deal foundered on Spanish league salary cap rules.
Barcelona, carrying debts of 1.2 billion euros ($A1.9 billion), are unable to register new signings because of their financial predicament.
With Abu Dhabi-owned Manchester City having ruled themselves out, PSG were about the only club who could afford what is expected to be a deal worth 35 million euros ($A55 million) a year.
“I gave everything for Barcelona from the first day that I arrived right to the last. I never imagined having to say goodbye,” Messi said on Sunday.
“I have still not come to terms with the reality of leaving this club now -- I love this club.”
He leaves Barcelona with 672 goals in 778 appearances, a record tally for one club.
Messi won 35 trophies at the Camp Nou after joining Barca aged 13, but his last appearance was a damp squib: a 2-1 home defeat behind closed doors against Celta Vigo in May.
His trophy haul includes four Champions Leagues and 10 La Liga titles.
“It will be difficult to see him in another shirt than that of Barca,” Messi’s old Barcelona team-mate Andres Iniesta, now playing in Japan, told AFP in an interview on Tuesday.
“As a player he just transcended everything. I’ve not seen a player like him -- and I don’t think I ever will. You look at what he did and for so long...”
Messi, who flew in to Paris with his wife and three children, is now preparing for a new life in Paris and in Ligue 1.
His arrival makes PSG even more obvious favourites to reclaim a French title they missed out on last season to Lille.
Some in Ligue 1 have mixed feelings about the impending arrival of arguably the greatest player of all time.
“It’s extraordinary for Ligue 1,” said Metz coach Frederic Antonetti.
“But for a purist like me, Messi should have finished his career at Barcelona.”
This summer PSG have already added veteran Spanish defender Sergio Ramos from Real Madrid and Italy goalkeeper Gianluigi Donnarumma, the star of Euro 2020.
They have also signed Liverpool midfielder Georginio Wijnaldum, snatching the Dutchman from under the noses of Barcelona, and spent 60 million euros on Inter Milan right-back Achraf Hakimi, who scored on his PSG league debut at the weekend.
An added attraction in moving to Paris is that coach Mauricio Pochettino, like Messi, started his career at Newell’s Old Boys in Rosario, Argentina.
Messi could be unveiled to supporters on Saturday, when PSG host Strasbourg and a full house of nearly 48,000 will be allowed in for the first time since the coronavirus pandemic struck 18 months ago.
Story first published on Fox Sports