"I made the decision that I'm going to go to Miami," Messi said in an interview with Mundo Deportivo and Sport newspapers.
"I still haven't closed it 100%. I'm still missing a few things, but we decided to go ahead. If Barcelona didn't work out, I wanted to leave Europe, get out of the spotlight and think more about my family."
Messi, who led Argentina to World Cup glory in Qatar in December and has earned a record seven Ballon d'Or awards, won the Ligue 1 title in his two seasons with PSG as well as the French Super Cup in 2022.
The move is also a big win for MLS, which welcomed Messi while adding that work remained to finalize the details of the formal agreement.
"The (goat) is coming," MLS tweeted, with a goat animal emoji standing in for the phrase "greatest of all time".
"Millions of MLS fans all over the world welcome you, Leo."
Messi had wanted to go to a club where he could eventually have an ownership stake, a source with knowledge of the negotiations told Reuters this week, and his contract is expected to pave the way for him to do so after he retires.
He will also receive a cut of the revenue from Apple TV's MLS Season Pass, which broadcasts the league's games, and be able to maximize his existing sponsorship deal with Adidas.
MLS earns a flat fee of around $250 million per year from Apple until it reaches a certain threshold of subscriptions, after which point it will earn a share of the revenue from those subscriptions.