Wiley agreed a six-year contract with the Premier League club with the option of a further one-season extension.
"Chelsea is pleased to announce the signing of United States international Caleb Wiley," the club confirmed on their website.
The left-back had been part of Atlanta's first team since 2022 and made his senior debut for the United States against Mexico last October.
Wiley is currently in Paris, where he is part of the US squad at the Olympic Games.
When he arrives at Stamford Bridge, Wiley will be assessed by new boss Enzo Maresca as competition for Marc Cucurella and Ben Chilwell, who are Chelsea's senior left-backs.
However, it is has been reported Wiley could spend next season on loan at French club Strasbourg, who are part of the same Blueco ownership group as Chelsea.
