Not since Brazilian legend Pele signed for New York Cosmos in 1975, has one of the all-time greats of the game played in the United States and his arrival is sure to set off an unprecedented level of hype.
It is hoped Messi, who won the World Cup with his country in December, will give a massive boost to the popularity of the league and the sport in the run up to the 2026 World Cup which the US is hosing along with Mexico and Canada.
While Pele's years in the old North American Soccer League were only seen by fans at the stadium and on the occasional live broadcast, Messi's matches in the pink shirt of Inter Miami will be available worldwide.
MLS's 10-year deal with Apple TV, worth a reported $2.5 billion, will allow fans in 107 countries to watch Messi in action via a subscription.
"Pele changed the sport in North America by introducing people to what soccer is," former U.S. international Taylor Twellman, now lead MLS analyst for Apple TV, told AFP. "Messi will be all that and then some because MLS has the infrastructure and the World Cup to take this rocket ship to the moon."
MLS and Apple will be hoping that the 35-year-old, seven-time Ballon d'Or winner's arrival will generate the kind of revenue that has so far eluded the 27-year-old league.
While MLS has grown to 29 clubs, with investors paying around $500 million for the next new team in San Diego, MLS remains far away from the levels of interest enjoyed by the NFL and NBA.
Apple TV along with MLS sponsors Adidas are reported to have played a key role in putting together the record deal which is certain to see Messi become the highest-earning player in the history of the game in the United States.
The last mega-deal for an international star that MLS pulled off was to bring David Beckham from Real Madrid to the LA Galaxy in 2007.
There is a direct link from that contract to Messi's deal -- Beckham was lured to MLS with the offer of an expansion team at a discounted rate and used that option to create Inter Miami, which entered the league three years ago.
Beckham remains a co-owner of Inter, which is currently bottom of the Eastern Conference and recently sacked head coach Phil Neville.
The club's majority owner, Jorge Mas will be well aware that Messi's local impact will turn his club from a sideshow to headline act in South Florida.
Calm unlikely. Miami has a large Argentinian community among its massive Latin American population and while Messi, on Wednesday, expressed a desire to have "more calm" in his life, that it is highly unlikely.
Mas and Beckham have spent five years trying to turn their dream of Messi in the Sunshine State into a reality.
Back in 2018, when Beckham announced that he finally had the go-ahead for his new team, Messi posted a social media video congratulating the Englishman.
"I wanted to wish you all the best in this new project, this new role for you," said the former Barcelona star. "Who knows, maybe in a few years you can give me a call."
The call arrived -- three years later, Mas told the Miami Herald that he and Beckham were working hard on a trying to attract the player to their club.
"I am optimistic Messi will play in an Inter Miami shirt because I think it will complete the legacy of the greatest player in our generation," said Mas.
Messi is understood to have held discussions with Mas before eventually deciding to sign for Paris Saint Germain in 2021.
Whatever was said in those talks, it didn't deter Mas and Beckham from continuing to pursue Messi.
Mas attended the World Cup in Qatar, where Messi was to score twice in the final and raise the trophy, meeting with the player's advisors.
In April, Beckham was photographed with Messi at PSG's training ground, setting off another wave of rumors.
Whenever pressed, club and MLS officials pointedly refused to deny their interest in Messi, ensuring the speculation continued and the hopes of the fans remained raised.
One person kept quiet throughout though -- Messi himself.
On Wednesday he ended that silence and the speculation and started what may be the final chapter in his career but could be the start of a new era for MLS.
© Agence France-Presse
Messi brings glory years in Europe to a close
About to turn 36, Lionel Messi could hang up his boots content that his glorious career is complete after he led Argentina to victory at last year's World Cup.
The Barcelona legend has secured his legacy as the best of his generation and perhaps now sits on the podium of all-time greats alongside the late Pele and Diego Maradona.
Yet he is clearly not ready to quit the game just yet, following the end of his two-year stay at Paris Saint-Germain.
That left Messi with a dilemma as he sought his next destination.
Almost every club would still take the seven-time Ballon d'Or winner, even the late-career version, in a heartbeat.
But signing Messi, whose annual salary in Paris was a reported 30 million euros ($32 million) after tax, comes at an enormous cost, leaving the player himself with only a handful of possibilities.
The romantic return to childhood club Newell's Old Boys, in his home city Rosario, will have to wait.
Barcelona wanted to bring him back and that would have been a love story in its own right too.
At one point a move to the Middle East seemed inevitable, but the pull of Major League Soccer proved too much to resist.
In making a move to the United States, "La Pulga" (The Flea) has done what Pele did in his mid-30s, and Johan Cruyff too.
Global icon. The last player who transcended the sport and made a move Stateside as a genuine global icon, was David Beckham, who was 32 when he joined LA Galaxy in 2007.
Back then Messi was emerging on the world stage, having made his senior Barcelona debut aged 17 in 2004.
Twice he played against Beckham in the Clasico, a fixture in which he scored a hat-trick while still a teenager in a 3-3 draw in March 2007.
Soccer has been transformed in the years that followed, while Messi established himself as one of the best to ever play the game.
The Gulf influence has been central to that transformation, and led to Messi moving to Qatar-owned PSG in 2021 when Barcelona could not afford a new contract for the seven-time Ballon d'Or winner.
And so he was a PSG player when he lifted the World Cup in Doha last December wearing a 'bisht,' a traditional Arab cloak that had been draped over him by Qatar's emir.
But Messi will not follow the path of his old rival, Cristiano Ronaldo, in moving to Saudi Arabia, which hopes to stage the World Cup in 2030.
Europe's leading domestic leagues, and the UEFA Champions League, remain the pinnacle of the club game, and it had been said Messi wanted to remain on the continent at least until the 2024 Copa America, when Argentina will defend that title.
Instead, the next stage of his career, and possibly the last, will come in the country that will host that tournament, and will jointly host the 2026 World Cup with Canada and Mexico.
One more major international honor might be the last thing motivating a man who has won it all over two decades, from the Under-20 World Cup in 2005 to the main trophy last year, when Argentina defeated France on penalties in the final.
"What more could there be after this," Messi asked after that game, in which he scored twice before also netting in the shootout that saw Argentina eventually prevail.
'Best player in history.' In that moment he emulated Maradona's own achievement in 1986, but nobody comes near to matching what Messi did at Barcelona.
He scored 672 goals in 778 appearances for Barca, winning the Champions League four times and La Liga on 10 occasions.
A darting, slightly injury-prone young winger -– who needed Barcelona to pay for growth hormone treatment as a teenager -– became a devastating 'false nine' and later the ultimate playmaker.
A lethal free-kick taker, the little man even strained his neck muscles to score a classic headed goal in the 2009 Champions League final against Manchester United.
By his later years he spent long periods of games walking around seemingly on the periphery, before bursting into life.
"I've said many times, for me, he's the best," reflected Pep Guardiola, his old Barcelona coach, in December. "If he had not won the World Cup, my opinion about what he has done for world football would not change."
If Messi at his peak was mesmerizing, the last two years at club level have been underwhelming.
He scored 32 goals in 75 games for PSG, set up countless goals for Kylian Mbappe and won two French titles.
However, he never seemed fully settled in Paris with his young family, and could not elevate the team in the Champions League.
The sense has been of a genius in decline, albeit one whose peak was arguably higher than anyone before him.
"I have had the privilege to coach the best player in the history of soccer," said PSG coach Christophe Galtier.
© Agence France-Presse