Gaga Slonina has joined Chelsea on a six-year contract before returning to the Chicago Fire on loan until January.
The 18-year-old has established himself as the most exciting young goalkeeper in MLS since making his Fire debut last August.
Then 17, Slonina became the youngest goalkeeper to start a match in MLS history as he kept a clean sheet in a draw with New York City FC.
Slonina has gone on to keep 13 clean sheets in 34 appearances and is Chicago's first-choice keeper, playing every minute of the 2022 campaign.
The United States youth international, who rejected a senior call-up from Poland in May, will play out the rest of the season in Illinois ahead of joining a pool of Chelsea goalkeepers that includes Edouard Mendy.
Chelsea have reportedly paid an initial £8.2million ($10m) for Slonina, although that fee could increase to £12.3m ($15m).
Honoured to join such a massive club with so much history. It’s a privilege to wear the badge of the best side in London.— GAGA (@GabrielSlonina) August 2, 2022
Thank you to all the fans for the warm welcome. Can’t wait to get started. @ChelseaFC pic.twitter.com/YmShsm1yYj
Fire sporting director Georg Heitz said in a statement: "We're very proud and happy to see Gaga reach this milestone moment in his career.
"Since joining the club, Gaga has embodied what it means to be a Chicago Fire player.
"In addition to his incredible talent, he is mature beyond his years, extremely hard-working and a fantastic team-mate.
"Gaga is a role model to many young aspiring footballers in Chicago, and this transfer demonstrates that a player can progress to the first team from our academy before joining one of the biggest clubs in the world."
Slonina added: "I joined this club [the Fire] with ambitions of playing at the highest level, and it's been a dream come true to wear the Chicago Fire jersey.
"This club has become my second home, my second family, and there are so many people that have played a big role in helping me get to where I am today.
"I'm so thankful for all the support that I've received from my family, coaches, team-mates and everyone associated with the club. It's been an incredible honour to work alongside each one of you."