Messi made the announcement about his next club in an interview with Spanish publications Mundo Deportivo and Sport. MLS later confirmed the proposed deal on social media.
The 2022 World Cup winner was linked with a return to former club Barcelona as well as a possible move to Saudi Arabian club Al Hilal, but ultimately decided to join co-owner David Beckham's Inter Miami in MLS.
"I have made the decision that I am going to Miami. I still don't have it agreed upon 100 percent and a few things are needed, but, well, we decided to continue my path there," Messi said.
"After winning the World Cup and not being able to return to Barcelona, it was my turn to go to the league of the United States to live football in another way and to enjoy the day-to-day more. Obviously, with the same responsibility and desire to play well and do things well as always. But in a calmer way."
Sources confirmed to ESPN's Jeff Carlisle that terms include an option for part-ownership of Inter Miami, but it would not be subsidized by the league as it was for Beckham in 2007. When Beckham moved to the LA Galaxy, he was given the option to purchase an MLS team for a discounted price of $25 million.
ESPN also confirmed that a cut of revenue from new subscribers to Apple TV's MLS Season Pass streaming service was part of negotiations. Any agreement involving Messi and Adidas would strictly be between the player and the company and wouldn't directly involve MLS.
"We are pleased that Lionel Messi has stated that he intends to join Inter Miami and Major League Soccer this summer," MLS said in a written statement. "Although work remains to finalize a formal agreement, we look forward to welcoming one of the greatest soccer players of all time to our League."
After Messi and Inter Miami made their announcements, Barcelona wished the Argentina star "the best of luck in his new professional phase" and said an offer was presented before Messi made the decision to move to MLS.
"Jorge Messi, the player's father and representative, informed Club President Joan Laporta of the player's decision to join Inter Miami, despite having been presented with a proposal from Barça, in consideration of the desire of both FC Barcelona and Lionel Messi for him to once again wear blaugrana," Barca said in a statement.
The Catalan club revealed the intention to work with Messi in the future "to promote a proper tribute from Barça fans to honor a footballer who has been, is, and always will be beloved by Barça."
The arrival of Messi, 35, promises to be the biggest signing in the 27-year history of MLS, rivaled only by the signing of Beckham.
His signing will also be a major boost for a Miami team that sits bottom of the Eastern Conference and last week dismissed coach Phil Neville.
Herculez Gomez says MLS is about to win big with the imminent arrival of Lionel Messi at Inter Miami.
Miami has reportedly held talks with Tata Martino about taking the managerial role there. Martino is a former manager for Argentina and for Barcelona, having worked with Messi on both teams.
Barcelona repeatedly stated their desire for Messi to return to the Catalan club, but sources told ESPN's Jordi Blanco that despite Barca's efforts to adjust their payroll to make the move work financially, Messi opted not to wait any longer and ruled out his former club as a destination for next season.
Messi began his career at Barcelona, joining as a 13-year-old youth player and ending as the club's record appearance-maker and top goal scorer, as well as winning 10 LaLiga trophies and four Champions League titles.
However, amid a financial crisis, Barca were unable to renew his contract in August 2021, forcing him to leave. He joined PSG on a free transfer later that month.
Alongside Kylian Mbappe and Neymar, he helped PSG to the Ligue 1 title in both seasons he spent in the French capital. However, back-to-back round-of-16 exits from the Champions League meant the club's search for a first European title continues.
He was booed by some fans on numerous occasions during his time in Paris, including in his final match for the club when he bid farewell.
Messi scored 32 goals in 75 games in all competitions for PSG, as well as recording 35 assists.
His stint at PSG came alongside other accomplishments, too. In December, Messi led Argentina to win the 2022 World Cup, ending his wait for the only major honour to have eluded him in his career.
He was then named men's footballer of the year at The Best FIFA Football Awards in February -- his second time winning the award, adding to his record seven Ballon d'Or trophies.