The 36-year-old striker who sparked Argentina to a World Cup title last year in Qatar, is set to be unveiled by the team in a Sunday ceremony and expected to join Inter Miami on the pitch as early as Friday.
"I'm very excited to start this next step in my career with Inter Miami and in the United States," Messi, who moves from Paris Saint-Germain, said in a statement.
The seven-time Ballon d'Or winner's arrival in Miami from Paris Saint-Germain has already caused a sensation and is expected to spark greater interest in the game and MLS across the United States.
It's also hoped the long-time Barcelona talisman can revive the fortunes of a Miami squad at the bottom of the MLS Eastern Conference at 5-13 with three drawn.
"This is a fantastic opportunity and together we will continue to build this beautiful project," Messi said. "The idea is to work together to achieve the objectives we set and I'm very eager to start helping here in my new home."
Messi's debut is expected on Friday when Inter Miami will host Cruz Azul in the Leagues Cup, a revamped competition between MLS and Mexican league squads.
It's the biggest boost for the North American League since English star David Beckham, now a co-owner of Inter Miami, joined the Los Angeles Galaxy in 2007. He launched the MLS squad in 2020 after years of trying to find a stadium site.
"Ten years ago, when I started my journey to build a new team in Miami, I said that I dreamt of bringing the greatest players in the world to this amazing city, players who shared the ambition I had when I joined LA Galaxy to help grow football in the USA and to build a legacy for the next generation in this sport that we love so much," Beckham said in a statement.
"Today that dream came true.
"I couldn't be prouder that a player of Leo's calibre is joining our club, but I am also delighted to welcome a good friend, an amazing person and his beautiful family to join our Inter Miami community.
"The next phase of our adventure starts here and I can't wait to see Leo take to the pitch."
'World's greatest'
Inter Miami head coach Gerardo "Tata" Martino has managed Messi in two separate prior stints – with FC Barcelona in the 2013-14 campaign, winning the 2013 Spanish SuperCup, and from 2014-2016 with the Argentine national team.
Maestro Messi captained the Albiceleste to victory at the 2021 Copa America as well as the Qatar World Cup and has records of 103 goals and 175 appearances for Argentina.
"We're overjoyed that the greatest player in the world chose Inter Miami and Major League Soccer," MLS commissioner Don Garber said.
"His decision is a testament to the momentum and energy behind our league and our sport in North America."
Messi is set for a glitzy arrival ceremony on Sunday at "The Unveil" with 18,000 expected to watch his first public event with his new club, followed by his first training session on Tuesday.
"We made a promise to build an ambitious club that would attract the world's elite players," Inter Miami managing owner Jorge Mas said.
"A heartfelt thank you to our fans that never stopped believing. Together we will continue to turn dreams into reality."
With iconic Messi expected to be joined by Spanish midfielder Sergio Busquets, the hope is Inter Miami can rise from a doormat to a champion.
"Lionel Messi is an incomparable talent," said Inter Miami sporting director Chris Henderson. "What he brings on and off the field will elevate everyone around him."
His legend has few rivals.
Messi is a two-time World Cup Golden Ball winner, a three-time UEFA Men's Player of the Year Award winner who also has six La Liga Best Player titles.
Messi is a four-time UEFA Champions League winner, an Olympic gold medal winner, has played on 10 La Liga champions, two Ligue 1 champions and taken seven Copa del Rey titles.
He played for Barcelona from 2004-2021 before joining Paris Saint-Germain for two seasons, making 75 appearances across all competitions, tallying 32 goals and 35 assists.
