Messi's contract became official on Saturday, a little more than five weeks after he declared that he would be coming to the club.
He'll be introduced on Sunday night at Inter's stadium in Fort Lauderdale, and the first home match of the Messi era could be as early as Friday in a Leagues Cup match against Cruz Azul.
A formal news conference is scheduled for Monday, and his first training session with his new club is expected on Tuesday.
The club previously announced that Messi's deal will be for two-and-a-half seasons and he'll be paid between $US50 million ($A73 million) and $US60 million ($A88 million) annually.
That puts the total contract value between $US125 million ($A183 million) and $US150 million ($A219 million) in cash alone.
"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."
It is, in many ways, an unusual marriage.
Messi — still perhaps the biggest star in world football — is signing with a team that entered Saturday in last place in the MLS's Eastern Conference standings.
It's a club in just its fourth season that has never won a championship and plays in a temporary home that will seat only about 22,000 people when some rushed renovations like adding extra bleacher seats get completed.
It doesn't matter. Soccer royalty now plays for Inter Miami.
"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," co-owner, former England superstar David 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."
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