On the day of his 35th birthday, Busquets has signed a contract until the end of 2025 after passing his medical, becoming the latest new player for David Beckham's Florida State franchise.
The former Barça and Spanish national team captain ruled out renewing his contract with Barcelona, rejected a million-dollar offer from Saudi Arabia and preferred, for family reasons, to follow in the footsteps of Leo Messi.
Busquets, who arrived in Miami on Sunday and will also be unveiled alongside Messi on Sunday evening, will wear the number 5 shirt - the number that has almost always accompanied him in his prolific career.
In Miami, he will link up not only with the Argentine star but also with Tata Martino, who coached him during the 2013/14 season at Barcelona.
Inter had announced last month that they were acquiring Busquets, who will be reunited with his former Barcelona teammate Messi.
The one-time World Cup champion, nine-time La Liga winner and former Barca captain will occupy a Designated Player slot and is expected to join the team "in the coming days," Inter said.
"I am excited to welcome Sergio Busquets to Inter Miami," managing owner Jorge Mas said in a statement.
"Since day one we have set out to bring the world's best players to Inter Miami. Sergio's repertoire speaks for itself."
Busquets, 34, said in May that he would leave Barcelona this year at the end of his contract.
He made 134 appearances for Spain before announcing his retirement from the team in December.
Busquets has been a central part of Barcelona's successes in the past 15 years, including three Champions League triumphs.
Busquets played as pivot behind Andres Iniesta and Xavi Hernandez in an iconic midfield trio, and the three of them also won the World Cup with Spain in 2010.
"He is one of the smartest players to ever play the sport," Inter's sporting director Chris Henderson said. "He reads the game at an unprecedented level and impacts every facet of play."
Busquets said the move offered a "special and exciting opportunity."
"I was impressed by the cub when I came with Barcelona last year and now I'm happy and ready to represent the club myself. I can't wait to help bring the success that this ambitious club strives for."
The signing comes a day after Messi inked the contract that has energized football fans in Miami and beyond, and on the same day that Inter planned to officially present him at an "unveiling" at the club's ground.
Messi and Busquets arrive as Inter Miami have sunk to the very bottom of Major League Soccer.
The club's winless run extended to 11 games with a 3-0 defeat at St. Louis City on Saturday -- their 14th defeat in 22 games.
Miami are bottom of the Eastern Conference and their latest loss also left them in last place in the overall standings -- 29th of 29 clubs.
Messi has been expected to make his debut on Friday against Cruz Azul in the new Leagues Cup, a tournament featuring all the top-flight teams from MLS and Mexico's Liga MX.
Inter said Busquets will first be available for the same game.
