The 22-year-old Mia Fishel joins Chelsea Women from Mexican club Tigres, with whom she won a league title last year.
Fishel received a first United States call-up in 2020 but has yet to make her international debut.
California-born Fishel, who has followed Chelsea from a young age, joined Tigres in January 2022, helping them to the Liga MX Femenil title in her first season while at the same time finishing as the league's top scorer.
The striker has also represented her country 28 times at youth international level, wining the Golden Ball at the Under-20 CONCACAF Championship in 2020.
She told Chelsea Women website: "I'm super excited, it's a dream come true and everything that I've worked for since I was a kid.
"It's all worth it and it's all paid off because now I'm playing for one of the best clubs in the world."
"I want to win titles, I want to win championships, I want to play in the Champions League and dominate. My ambition, just like this club, is to be the best."
And general manager Paul Green added of the USA U20 international: "Mia is a born goal scorer and we think that she's at a great age to take the next step in her career. We're really excited and feel that her pace and power is going to be a big asset for the team in the upcoming years."
Fishel was top scorer as Tigres won the Apertura title in the Liga MX Femenil last season, with the Monterrey-based team taking the honours for the opening half of the campaign that is split into two championships.