Messi, who officially joins Miami after leaving French champions Paris Saint-Germain following two seasons at the club, has signed a contract through the end of the 2025 MLS season.
Inter Miami managing owner Jorge Mas, who said it took three years of negotiations to bring the 36-year-old to the club, has stated that Messi will earn "between £50 million and $60m per year."
"I'm very excited to start this next step in my career with Inter Miami and in the United States," Messi said. "This is a fantastic opportunity and together we will continue to build this beautiful project.
"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."
Miami confirmed that Messi will first be available for selection in a July 21 Leagues Cup match against Mexican side Cruz Azul.
Saturday's announcement follows an interview Messi gave to Spanish outlets June 17 when he stated his intention to move to the MLS club, rejecting a possible return to Barcelona as well as a huge financial offer to become the latest big name to head to Saudi Arabia.
Sources told ESPN that terms of the deal include an option for part-ownership of Inter Miami, but it would not be subsidized by the league as it was for David Beckham in 2007. When Beckham -- now a co-owner of Inter Miami -- 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.
Messi will occupy one of three designated player spots on Miami's roster after the club created an opening by terminating the contract of Mexican midfielder Rodolfo Pizarro on Friday.
Sí, Muchachos📍 pic.twitter.com/8E3f9hb9VU— Inter Miami CF (@InterMiamiCF) July 15, 2023
"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," Beckham said. "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.
"Today that dream came true. I couldn't be prouder that a player of Leo's caliber 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."
Messi is set to be joined in Miami by former Barcelona teammates Sergio Busquets and Jordi Alba and will also be reunited with former Barcelona and Argentina coach Gerardo "Tata" Martino, who took over as manager in late June.
"We are overjoyed that the greatest player in the world chose Inter Miami CF and Major League Soccer, and his decision is a testament to the momentum and energy behind our League and our sport in North America," MLS commissioner Don Garber said in a statement.
"We have no doubt that Lionel will show the world that MLS can be a League of Choice for the best players in the game. We look forward to seeing his debut for Inter Miami in our Leagues Cup tournament later this month."
Messi has won every major honor available to him during a legendary career that puts him among the greatest players of all time. As well as being awarded the prestigious Ballon d'Or as the world's best player a record seven times, Messi has lifted 43 titles.
Thirty-five of those came with Barcelona: including 10 LaLiga titles, eight Spanish Supercopas, seven Copa del Reys and four Champions Leagues. He left the Catalan club in summer 2021 as its record scorer with 672 goals.
With Barcelona submerged in financial trouble, Messi was forced to depart and spent the past two seasons at PSG, where he won Ligue 1 titles both years.
Perhaps his greatest moment came just six months ago, when he led Argentina to its first World Cup title since the Diego Maradona-inspired team of 1986. Messi scored twice in the final as Argentina beat France in a penalty shootout following a dramatic contest in Qatar.
Messi will be faced with another major challenge when he begins life for his new club, with Miami sitting bottom of the Eastern Conference after going 10 MLS games without a win.