CHICAGO: Goalkeeper Gabriel Slonina was acquired by Chelsea from the Chicago Fire, then loaned back to the Major League Soccer team for the rest of 2022.
Chelsea said he had signed a six-year contract.
Chicago claimed the transfer fee for the 18-year-old was its club record, but did not announce the amount.
The Fire said they will receive a percentage of future transfer fees.
Born in Addison, Illinois, Slonina became MLS's youngest starting goalkeeper in August 2021 at 17 years, 81 days, in a 0-0 draw against New York City.
He started 11 league matches last season, including the final 10, and has started 23 league matches this season.
Slonina attended U.S. national team training camps in December and January, but did not get into any matches.
He has played for the U.S. at several youth national team levels.