PARIS — When Lionel Messi said goodbye to Barcelona, his home since childhood and the place he grew to become one of soccer’s greatest ever players, he was in tears.

Three days later, when he was formally introduced on Wednesday by his new club, Paris St.-Germain, any tears in the crowd were expressions of joy.

And Messi, carrying his new jersey, traded the heavy emotions for a light smile.

“I still want to play and I still want to win,” he said at a news conference on Wednesday, sitting next to P.S.G.’s president, Nasser al-Khelaifi. Leaving Barcelona was “a very hard moment,” he said, but he added that he was “very happy” to be in Paris.

“I want to keep growing and keep winning titles.”

Messi’s signing with P.S.G. on Tuesday represented the most significant — if unsurprising — move in recent soccer history, one that countless fans, both in France and Spain, were still struggling to fathom even as Messi walked on the field at the Parc des Princes, P.S.G.’s stadium and his new home.

The move will reinforce P.S.G.’s dominance over France’s domestic league, Ligue 1, which has struggled to attract attention in recent years. And it will make the club the overwhelming favorite for the Champions League, the only trophy that matters to its Qatari backers, one that it has never held and that its president dreams of winning before Qatar hosts the World Cup in December 2022.

“We haven’t won anything,” al-Khelaifi said, ignoring the seven Ligue 1 titles the club has claimed in the last nine seasons. “This is just the start.”

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Messi, speaking alongside Paris St.-Germain’s president, Nasser al-Khelaifi, said leaving Barcelona was “a very hard moment,” but that he was “very happy” to join P.S.G. Credit...Stephane De Sakutin/Agence France-Presse — Getty Images

Outside the stadium, fans chanted and threw smoke bombs as they waited to catch a glimpse of the Argentine star.

“It’s wonderful,” said Alexandre Marienne, 32, carrying his 8-year-old son, Kamil, on his shoulders. “He’s going to help us build something incredible — Paris is definitely competing with the big names now.”

It was the culmination of a stunning few days, in which Barcelona’s fans and players bid farewell in shock to the club’s greatest player, while in the French capital, P.S.G.’s fans held their breaths.

Before flying to Paris, Messi had repeated that he didn’t want to leave the club that made him who he is, that he had done “everything to stay” in Barcelona. His devoted fans wanted him to stay, too. The club wanted him to stay. On Wednesday, Messi said it would be weird to go back to his home with a different jersey if he were to play against his former club.

But the financial forces that drive the game were greater than either individual or collective desire. Barcelona could not afford Messi, even after he offered to cut his salary in half.

So here he was, in Paris, about to play in the French Ligue 1, where financial rules akin to those that tied Barcelona’s hands will not come into force for a few more years.

“The moment I arrived here, I felt very happy,” Messi said at the news conference before greeting the fans outside. He is joining a roster full of stars already, one that fans — until now — could only dream of assembling in a video game. This summer, P.S.G. added to its squad the Italian goalkeeper and recent European Championship winner Gianluigi Donnarumma, the Dutch midfielder Georginio Wijnaldum and the Spanish defender Sergio Ramos — all free agents, like Messi.

Yet seeing Messi with other colors than Barcelona’s, something unthinkable until last week, remained an oddity even as he threaded the field of the Parc des Princes on Wednesday. He will don No. 30, which he had at Barcelona from 2004 to 2006. Neymar, the Brazilian who has been P.S.G.’s biggest international name until now, will keep No. 10.

Rarely has an athlete in the modern era been so associated with a single team — Messi moved to Barcelona when he was only 13 — and P.S.G. is likely to be his last club in Europe.

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Messi wept during his last news conference in Barcelona, Spain, on Sunday. Credit...Albert Gea/Reuters

But the legions of fans who greeted Messi in his new home city opened their arms in an embrace that, for the moment, overshadowed the darker message his transfer sent about the sport that he has so dominated, and the nostalgia felt in Spain.

These fans had not come to discuss the danger posed by the immense advantage a small number of superrich clubs now have in buying and keeping players.

They had simply come to see Messi.

Men and women, many with their children by their side, came from all over Paris and other French cities far and near. Some were not from France at all. But on Wednesday, they were all bonded by Messi.

Messi called the ecstatic reception “crazy,” and said he was excited to get back to the business of playing soccer with some of the best players in the world. With Neymar and Kylian Mbappé by Messi’s side, P.S.G. will have Europe’s best attack, although it remains to be seen how Mauricio Pochettino, the club’s manager, will make so many stars fit together.

Messi’s move is casting a shadow on the future of Mbappé, the 22-year-old French superstar who grew up near Paris and whose contract with P.S.G. expires next year. But for al-Khelaifi, there was only one star worth talking about on Wednesday. “Kylian is a Parisian player. He says he’s a competitor,” al-Khelaifi said. “We can’t be more competitive than this team. He has no excuse to do something else.”

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Messi arrived in France on Tuesday and stayed at a palace in central Paris. Credit...Sarah Meyssonnier/Reuters

Since Qatar became the main stakeholder of Paris St.-Germain in 2012, supporters have watched the coming of a steady stream of the world’s most expensive players, from Zlatan Ibrahimovic to Neymar, David Beckham to Mbappé, Gianluigi Buffon to Ramos. No club has signed as many stars in the past 10 years.

That has drawn criticism from countless clubs, players and managers in France and abroad who argue that the competition is now unfair and biased toward state-sponsored teams like P.S.G. and Manchester City.

But none of those previous deals, it seems, compares with Messi’s arrival, and countless supporters could still not believe it.

“It’s just crazy stuff — we were not even dreaming of it,” said Yohan Aymon, a 19-year-old P.S.G. fan and forward for F.C. Sion, a Swiss club, who drove from his native Switzerland overnight.

“He made football magic, beautiful, and he’s a winner,” al-Khelaifi said as he stood next to Messi at the news conference. “There’s no secret he’s the best player in the world.”

Al-Khelaifi dismissed suspicions around financial fair play and said the club had always respected the rules. “We had the capacity to sign Messi for Financial Fair Play and we will always follow the rules,” he said. “I hope Leo will not ask for a bigger salary.”

Messi will earn 35 million euros a season, or about $41 million.

“We are entering a new dimension,” said Marienne, the 32-year-old fan, who said he had moved his vacation in southern France with his family to see Messi. “P.S.G.’s possibilities seem unlimited now.”