The 36-year-old announced last month that he had agreed a move to the American outfit after running his contract down at Paris Saint-Germain.
And now Inter Miami have unveiled their latest signing, with Messi to wear the No.10 jersey.
Inter took to social media to announce his arrival, with a video showing Messi's name spray painted in pink - the colour of the club's home shirt.
The Mirror have stated that Messi has penned a two-and-a-half year deal at the DRV PNK Stadium.
Messi has declared that Inter's project persuaded him to join, as he spoke of his delight at finally putting pen to paper.
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He said: "I'm very excited to start this next step in my career with Inter Miami and in the United States.
"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."
Inter chief Beckham added: "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.
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"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 will be the highest-profile player to play in the MLS - ahead of Inter co-owner David Beckham, Thierry Henry, Zlatan Ibrahimovic, Kaka, Steven Gerrard and Frank Lampard.
Miami fended off interest from Saudi Arabia and Barcelona to land the Blaugrana legend.
It has been claimed Messi has agreed a £41.2million-a-year contract with Miami, with the global star said to also receive shares of Apple and Adidas' revenue.
Messi will be presented to the club's supporters on Sunday - alongside ex-Barcelona teammate Sergio Busquets.
The duo, who could make their debut against Mexico's Cruz Azul in the Leagues Cup next week, will be managed by former Barca boss Gerardo Martino.