The English Women's Super League champions announced the signing on Sunday, with Girma presented to the 34,302 crowd at Stamford Bridge before their league game against Arsenal, which Chelsea won 1-0.
The fee tops the 685,000 euros ($A1.1 million) that the NWSL's Bay FC paid for Zambia's Racheal Kundananji in February 2024.
Girma joins from the San Diego Wave, where she was under contract until 2026.
"There are a lot of things about Chelsea that made me want to come here -- the culture, the winning mentality, staff and players. It's a top environment to learn and grow in," Girma said in a club statement.
The 24-year-old has been hailed by her US team manager and former Chelsea boss Emma Hayes as the world's best defender, having played every minute of the 2024 Paris Olympic tournament where the Americans won gold with a 1-0 victory over Brazil in the final.
Girma, manager Sonia Bompastor's first January signing at the club, will help to shore up a Chelsea back line that lost centre-back Kadeisha Buchanan to a torn anterior cruciate ligament in November.
Holders Chelsea have a nine-point lead over Manchester City atop the WSL table, and their next fixture is away to Aston Villa on February 2.
The Wave selected Girma with the first pick of the 2022 NWSL draft and she went on to win both the league's top defender and rookie of the year awards that season.
She led the Wave to the NWSL Shield in the 2023 season and was named the league's top defender for the second successive year.
HOW THE RECORD TRANSFER FEE IN THE WOMEN'S GAME HAS INCREASED THIS CENTURY:
2002: Milene Domingues (Brazil) - Fiammamonza in Italy to Rayo Vallecano in Spain for $A 491,000.
2020: Pernille Harder (Denmark) - Wolfsburg in Germany to Chelsea in England for $A562,000.
2022: Keira Walsh (England) - Manchester City in England to Barcelona in Spain for $A812,000.
2024: Mayra Ramirez (Colombia) - Levante in Spain to Chelsea for $A858,000.
2024: Rachael Kundananji (Zambia) - Madrid CFF in Spain to Bay FC in the US for $A1.1 million.
2025: Naomi Girma (USA) - San Diego Wave in US to Chelsea for $A1.8 million.
